ci: add git to msys2 packages for Windows CI

go mod download needs git in PATH; MSYS2 shell doesn't inherit
the Windows system git.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

.github/workflows/ci.yml

@@ -60,7 +60,7 @@ jobs:
         with:
           msystem: MINGW64
           update: true
-          pacboy: gcc:p go:p
+          pacboy: gcc:p go:p git:p
 
       - name: Download dependencies
         run: go mod download