Support configuring the default build.cmd/.sh parameters using an environment variable

[eiriktsarpalis]

When building this repo with an -arch parameter unspecified, the default value will be resolved from the architecture of the currently running process:

runtime/eng/build.ps1

Line 14 in 5c53e8e

[ValidateSet("x86","x64","arm","arm64","wasm")][string[]][Alias('a')]$arch = @([System.Runtime.InteropServices.RuntimeInformation]::ProcessArchitecture.ToString().ToLowerInvariant()),

I use bash on Windows ARM which for the moment doesn't have a native arm64 build. For this reason, I always need to explicitly specify and -arch arm64 parameter to build the runtime using the native architecture. It would be really nice if we could configure the default build architecture using a dedicated environment variable.

EDIT See #104033 (comment) for a workaround.

cc @ViktorHofer

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[vcsjones]

+1. I have the same situation with SSHing in to WoA because the SSH DevTunnel agent uses x86 emulation on ARM.

[eiriktsarpalis]

Here's a sketch implementation:

eiriktsarpalis@f2c4e6b

Do we need this for build.sh? The default is being resolved somewhat differently there and I'm assuming there isn't an explicit need to configure this.

[eiriktsarpalis]

Any suggestions on the name? I started with DOTNET_RUNTIME_REPO_DEFAULT_ARCH but not aware of any prior art in naming convention used for environment variables in infrastructure.

[eiriktsarpalis]

Similar to #104548 (comment) one workaround is using the registry to set a preferred architecture for powershell.exe:

• HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\powershell.exe\PreferredMachine

as a DWORD set to the hex value of aa64.

[jkotas]

Updated title to reflect more general design discussed in #104474 : Introduce DOTNET_RUNTIME_BUILD_ARGS environment that can be used to specify defaults for all build.cmd/.sh parameters that can be specified just once. For example DOTNET_RUNTIME_BUILD_ARGS=-arch arm64 -c Release would make the build.cmd/.sh scripts to produce release build for arm64 architecture by default.