Momentarily downgrade Git on Cygwin to investigate failures

Using this older version is not in general secure, since the new
version is a security update. It is sometimes acceptable to run
software with security bugs in CI workflows, but the intent of this
change is just to check if the version of the Cygwin `git` package
is the cause of the failures. If so, they can probably be fixed or
worked around in a better way than downgrading. (Furthermore, the
lower version of the `git` package will not always be avaialable
from Cygwin's repositories.)

.github/workflows/cygwin-test.yml

@@ -30,7 +30,7 @@ jobs:
     - name: Set up Cygwin
       uses: egor-tensin/setup-cygwin@v4
       with:
-        packages: python39=3.9.16-1 python39-pip python39-virtualenv git
+        packages: python39=3.9.16-1 python39-pip python39-virtualenv git=2.43.0-1
 
     - name: Arrange for verbose output
       run: |