Upgrade Git in test-fixtures-windows ARM job

The `windows-11-arm` runner still has Git for Windows 2.48.1. It
thus remains affected by GitoxideLabs#1849 (git-for-windows/git#5436) for now.
This is responsible for failures seen so far on `windows-11-arm`,
other than the failures that appear to be related to performance.

This commit restores the pre-GitoxideLabs#1920 logic for upgrading the Git for
Windows installation on the runner, but does so *only* in the
`windows-11-arm` job, and *not* in the `windows-latest` job. The
code is modified to download the ARM64 installer rather than the
x86-64 installer.

Besides being only done for `windows-11-arm` and installing an
ARM64 build, this is the same as the code in GitoxideLabs#1892 (building
on GitoxideLabs#1870). It thus attempts to install the latest stable release,
but not pre-releases. See 3501e82 (GitoxideLabs#1920) for further background.

Especially in view of the preview status of the `windows-11-arm`
image, it may be a good idea to inspect and report the status of
the Git for Windows installation before and after the upgrade. But
this commit does not add such functionality.

Author: EliahKagan

.github/workflows/ci.yml

@@ -263,6 +263,36 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
+      - name: Upgrade Git for Windows to latest stable release
+        # TODO: Remove this step once `windows-11-arm` ships a new enough version of Git.
+        # (The Windows 11 ARM runner still ships Git 2.48.*, so it remains affected by #1849.)
+        if: matrix.os == 'windows-11-arm'
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          $workingDir = '~/git-dl'
+          $repo = 'git-for-windows/git'
+          $pattern = 'Git-*-arm64.exe'
+          $log = 'setup-log.txt'
+          # Inno Setup args reference: https://jrsoftware.org/ishelp/index.php?topic=setupcmdline
+          $arguments = @(
+            '/VERYSILENT',
+            '/SUPPRESSMSGBOXES',
+            '/ALLUSERS',
+            "/LOG=$log",
+            '/NORESTART',
+            '/CLOSEAPPLICATIONS',
+            '/FORCECLOSEAPPLICATIONS'
+          )
+
+          mkdir $workingDir | Out-Null
+          cd $workingDir
+          gh release download --repo $repo --pattern $pattern
+          $installer = Get-Item $pattern
+          Start-Process -FilePath $installer -ArgumentList $arguments -NoNewWindow -Wait
+
+          Get-Content -Path $log -Tail 50
+          git version
       - uses: actions/checkout@v4
       - name: Install rustup
         if: matrix.os == 'windows-11-arm'