Release pipeline: route vcpkg through Terrapin asset cache

[tyrielv]

Problem

The official release pipeline has been failing since libgit2 was
statically linked via vcpkg, because the pool's build agents do not have
direct public-internet access. vcpkg was reaching out to sourceforge.net
for libgit2 transitive dependencies (pcre, zlib, http_parser, ...) which
the network layer blocked, causing WinHttpSendRequest 12029 errors.

Solution

Route vcpkg asset downloads through the internal Terrapin cache (the
canonical CI asset-caching service). This is opt-in — external
contributors and the public GitHub Actions PR-validation workflow
(.github/workflows/build.yaml) continue to use the standard vcpkg flow
with direct downloads from public origins. No internal feed
authentication is required to build VFS for Git from a fresh clone, and
the repo source code intentionally contains no msbuild SDK references
that would pull internal packages into a default restore.

Changes

• Directory.Build.props: add IsLocalBuild and
  UseTerrapinAssetCache properties, both defaulted to false.
  IsLocalBuild controls Terrapin upload-on-fetch (allowed on dev boxes
  only); UseTerrapinAssetCache opts the build into the Terrapin
  asset-cache machinery (release pipeline only).

• Directory.Build.targets: build a --x-asset-sources string that
  combines the Terrapin retrieval script with x-block-origin, gated on
  UseTerrapinAssetCache=true, and append it to both vcpkg install
  invocations. Add a guard <Error> that requires the pipeline to
  supply TerrapinRetrievalToolPath up front when asset caching is
  enabled. Also validate static git2.lib (was only checking dynamic
  git2.dll).

• .azure-pipelines/official-release-nuget.config (new):
  pipeline-only NuGet config that points at an internal feed. Used
  solely by a one-off nuget install of Microsoft.Build.Vcpkg, not by
  any csproj restore. Not consulted by external contributors or by
  GitHub Actions.

• .azure-pipelines/release.yml:

  • Add NuGetAuthenticate@1 task.
  • Add a NuGetCommand@2 task that out-of-band installs the internal
    Microsoft.Build.Vcpkg package (which ships
    TerrapinRetrievalTool.exe) into $(Agent.TempDirectory).
  • Add an explicit Restore vcpkg native libraries (Terrapin cache)
    step that invokes _RestoreVcpkgDependencies on
    GVFS.Common.csproj with -p:UseTerrapinAssetCache=true and
    TerrapinRetrievalToolPath pointing at the extracted tool. The
    existing Build.bat invocation downstream then skips the vcpkg
    install because the libs are already present.

Verification

Verified with a successful run of the build stage in the official
release pipeline. End-to-end green: Restore vcpkg native libraries,
Build (Release), unit tests, installer build, and all 4 pipeline
artifacts (Installer, FastFetch, Symbols, FunctionalTests). The
Terrapin cache had every libgit2 transitive dep already seeded, so no
follow-up cache-seeding request is needed.