Upgrade GfW in test-fixtures-windows too, without duplication

GitHub Actions has recently started supporting YAML anchors:

- https://github.blog/changelog/2025-09-18-actions-yaml-anchors-and-non-public-workflow-templates/
- https://docs.github.com/en/actions/reference/workflows-and-actions/reusing-workflow-configurations#yaml-anchors-and-aliases

This uses that to have the same step in two separate job
definitions without duplicating the code itself.

It would often be preferable do put the job in a composite action,
but as noted in previous commit messages (especially in the
parenthesized paragraph at the end of 9e4e3ec), this job is best
run before `actions/checkout`, yet retrieving the action definition
from the current repository at the same ref (in a way that is not
excessively complicated or slow) requires that `actions/checkout`
have been used first. So this uses a YAML anchor instead.

Author: EliahKagan

.github/workflows/ci.yml

@@ -272,7 +272,8 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-      - name: Upgrade Git for Windows to latest stable release
+      - &upgrade_git_for_windows
+        name: Upgrade Git for Windows to latest stable release
         if: startsWith(matrix.os, 'windows')
         env:
           GH_TOKEN: ${{ github.token }}
@@ -379,6 +380,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
+      - *upgrade_git_for_windows
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           persist-credentials: false