ci(vs-build): adapt to Visual Studio 2026 default on windows-latest

The `windows-latest` runner image migration that began on June 8, 2026
and completes on June 15, 2026 switches the default Visual Studio
install from VS 2022 (v17) to VS 2026 (v18), per
https://github.com/actions/runner-images/issues/14017.

CMake 4.x picks up the new generator name "Visual Studio 18 2026"
automatically and, crucially, writes the solution file with the new
`.slnx` (XML) extension rather than `.sln`. See
https://github.com/Kitware/CMake/blob/v4.3.2/Source/cmGlobalVisualStudioGenerator.cxx#L1147-L1159
where `GetSLNFile()` appends an "x" to the filename when the generator
version is `VS18` or newer.

As a result, the `MSBuild` step in the `vs-build` job fails with

	MSBUILD : error MSB1009: Project file does not exist.
	Switch: git.sln

because the file CMake actually wrote is `git.slnx`. An example of the
failure can be seen at
https://github.com/git-for-windows/git/actions/runs/27264770241/job/80556419519.

Teach the step to prefer `git.slnx` and fall back to `git.sln` so that
it works on both the new image and any runner still on VS 2022 during
the week-long staggered rollout. The conditional is written in
PowerShell rather than bash so the step stays on the default shell:
`microsoft/setup-msbuild@v3` adds `msbuild` to the Windows `PATH` only,
and an MSYS2 bash spawned by the SDK does not pick it up (an earlier
attempt at this fix using `shell: bash` failed with
`msbuild: command not found`, see
https://github.com/git-for-windows/git/actions/runs/27290221733/job/80608493655).
Letting MSBuild itself discover the solution by omitting the project
argument is not an option here either: CMake emits all `*.vcxproj`
files (one per `add_executable`/`add_library`, e.g.
`git-daemon.vcxproj`, `common-main.vcxproj`, `ALL_BUILD.vcxproj`, ...)
into the same build root as the solution file, and MSBuild's
auto-discovery in
`ProcessProjectSwitch()` (`dotnet/msbuild`, `src/MSBuild/XMake.cs`)
rejects that combination as `AmbiguousProjectError` because it only
disambiguates the special case of exactly two projects where one has a
`.proj` extension.

Additionally, drop the `-property:PlatformToolset=v142` argument that
had been carried since 889cacb6 (ci: configure GitHub Actions for
CI/PR, 2020-04-11), when this job was first configured for VS 2019.
The VS 2026 install on `windows-latest` only ships the v144 toolset
along with a v143 compatibility component
(`Microsoft.VisualStudio.Component.VC.14.44.17.14.x86.x64`); v142 is
no longer present, so the explicit pin would now also fail in its own
right. Removing it lets MSBuild use whatever toolset CMake selected
during configuration (v143 on a VS 2022 runner, v144 on a VS 2026 one),
which keeps the configure and build steps consistent with each other
regardless of which image picked up the job.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Assisted-by: Opus 4.7

.github/workflows/main.yml

@@ -199,7 +199,9 @@ jobs:
         cmake `pwd`/contrib/buildsystems/ -DCMAKE_PREFIX_PATH=`pwd`/compat/vcbuild/vcpkg/installed/${{ matrix.arch }}-windows \
         -DNO_GETTEXT=YesPlease -DPERL_TESTS=OFF -DPYTHON_TESTS=OFF -DCURL_NO_CURL_CMAKE=ON -DCMAKE_GENERATOR_PLATFORM=${{ matrix.arch }} -DVCPKG_ARCH=${{ matrix.arch }}-windows -DHOST_CPU=${{ matrix.arch }}
     - name: MSBuild
-      run: msbuild git.sln -property:Configuration=Release -property:Platform=${{ matrix.arch }} -maxCpuCount:4 -property:PlatformToolset=v142
+      run: |
+        $sln = if (Test-Path git.slnx) { 'git.slnx' } else { 'git.sln' }
+        msbuild $sln -property:Configuration=Release -property:Platform=${{ matrix.arch }} -maxCpuCount:4
     - name: bundle artifact tar
       shell: bash
       env: