Pin pkgconf v2.1.0

pkgconf v2.1.1 has a regression with producing the right link order
for abseil (pkgconf/pkgconf#343). Pin
the version to v2.1.0 until that is resolved.

.github/workflows/tests.yml

@@ -146,6 +146,10 @@ jobs:
         with:
           name: cruby-gem
           path: gems
+      - name: Download pkgconf 2.1.0
+        run: curl -O https://repo.msys2.org/mingw/x86_64/mingw-w64-x86_64-pkgconf-1~2.1.0-1-any.pkg.tar.zst
+      - name: Install pkgconf 2.1.0
+        run: pacman -S mingw-w64-x86_64-pkgconf-1~2.1.0-1-any.pkg.tar.zst
       - name: Show pkg-config version
         run: pkg-config --version
       - run: ./scripts/test-gem-install gems