TEMPORARY: Overlay custom MSBuild for dotnet/msbuild#12927 investigation

[ilonatommy]

DO NOT MERGE - Investigation branch only

Purpose

Overlay custom Microsoft.Build.Tasks.Core.dll from the MSBuild team to diagnose intermittent MSB3030: Could not copy file errors during parallel CI builds.

Tracking issue: dotnet/msbuild#12927
Known Build Error: #62807

hits from the known build issue happened on:

Job	Step	Platform
Test: Ubuntu x64	Run build.sh	Linux
Build: Windows x64/x86/arm64	Run build.cmd	Windows
Build: Windows x64/x86/arm64	Build Installers	Windows
(internal) dotnet-aspnetcore	Run build.sh	Linux
(internal) dotnet-aspnetcore	Build Installers	Windows

So it hits both Linux and Windows build legs, specifically during the large parallel compilation steps (Run build.sh, Run build.cmd, Build Installers). These are the jobs where /m parallelism triggers the race condition. Our overlay covers both build.sh and build.ps1, so we're
covered on all the failing legs.

❯ are these legs running on our playground PR?

What this does

1. Commits a custom Microsoft.Build.Tasks.Core.dll (with extra logging) to eng/msbuild-bootstrap
2. After SDK initialization in eng/build.sh and eng/build.ps1, overlays the custom DLL onto .dotnet/sdk<version>/Microsoft.Build.Tasks.Core.dll
3. Prints SHA256 hashes to the build log so we can verify the overlay worked

Expected outcome

• Build logs will show === MSBuild Bootstrap Overlay ===section confirming the DLL was replaced
• If the MSB3030 error occurs, the binlog will contain additional diagnostic information from the custom MSBuild
• The binlog artifact should be shared with the MSBuild team (@OvesN) for analysis

How to revert

Close this PR. No repo-wide changes are needed.

This PR should be updated automatically when my devbox's task scheduler detects that CI finished without error that we're waiting for.

[ilonatommy]

/azp run aspnetcore-ci

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[ilonatommy]

/azp run aspnetcore-ci

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[ilonatommy]

🚨 MSB3030 "File Not Found" Error Detected (Automated)

Build: #1365043

Errors found:

• [Run build.sh] 2026-04-03T03:09:32.6419596Z ##[error].dotnet/sdk/11.0.100-preview.1.26104.118/Microsoft.Common.CurrentVersion.targets(5094,5): error MSB3030: (NETCORE_ENGINEERING_TELEMETRY=Build) Could not copy the file "/mnt/vss/_work/1/s/artifacts/bin/Microsoft.Net.Http.Headers/Release/net11.0/Microsoft.Net.Http.Headers.pdb" because it was not found.
• [Run build.sh] 2026-04-03T03:09:32.6428055Z /mnt/vss/_work/1/s/.dotnet/sdk/11.0.100-preview.1.26104.118/Microsoft.Common.CurrentVersion.targets(5094,5): error MSB3030: Could not copy the file "/mnt/vss/_work/1/s/artifacts/bin/Microsoft.Net.Http.Headers/Release/net11.0/Microsoft.Net.Http.Headers.pdb" because it was not found. [/mnt/vss/_work/1/s/src/Middleware/Localization/test/UnitTests/Microsoft.AspNetCore.Localization.Tests.csproj]
• [Run build.sh] 2026-04-03T03:16:00.7069704Z /mnt/vss/_work/1/s/.dotnet/sdk/11.0.100-preview.1.26104.118/Microsoft.Common.CurrentVersion.targets(5094,5): error MSB3030: Could not copy the file "/mnt/vss/_work/1/s/artifacts/bin/Microsoft.Net.Http.Headers/Release/net11.0/Microsoft.Net.Http.Headers.pdb" because it was not found. [/mnt/vss/_work/1/s/src/Middleware/Localization/test/UnitTests/Microsoft.AspNetCore.Localization.Tests.csproj]
• [Test: Ubuntu x64] 2026-04-03T03:09:32.6419631Z ##[error].dotnet/sdk/11.0.100-preview.1.26104.118/Microsoft.Common.CurrentVersion.targets(5094,5): error MSB3030: (NETCORE_ENGINEERING_TELEMETRY=Build) Could not copy the file "/mnt/vss/_work/1/s/artifacts/bin/Microsoft.Net.Http.Headers/Release/net11.0/Microsoft.Net.Http.Headers.pdb" because it was not found.
• [Test: Ubuntu x64] 2026-04-03T03:09:32.6428075Z /mnt/vss/_work/1/s/.dotnet/sdk/11.0.100-preview.1.26104.118/Microsoft.Common.CurrentVersion.targets(5094,5): error MSB3030: Could not copy the file "/mnt/vss/_work/1/s/artifacts/bin/Microsoft.Net.Http.Headers/Release/net11.0/Microsoft.Net.Http.Headers.pdb" because it was not found. [/mnt/vss/_work/1/s/src/Middleware/Localization/test/UnitTests/Microsoft.AspNetCore.Localization.Tests.csproj]
• [Test: Ubuntu x64] 2026-04-03T03:16:00.7069715Z /mnt/vss/_work/1/s/.dotnet/sdk/11.0.100-preview.1.26104.118/Microsoft.Common.CurrentVersion.targets(5094,5): error MSB3030: Could not copy the file "/mnt/vss/_work/1/s/artifacts/bin/Microsoft.Net.Http.Headers/Release/net11.0/Microsoft.Net.Http.Headers.pdb" because it was not found. [/mnt/vss/_work/1/s/src/Middleware/Localization/test/UnitTests/Microsoft.AspNetCore.Localization.Tests.csproj]

Action required: Collect binlog from this build and share with MSBuild team (@OvesN on dotnet/msbuild#12927).

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