chore: CI: switch to GCC on Windows for now

See discussion at https://github.com/leanprover/lean4/pull/555

.github/workflows/ci.yml

@@ -47,7 +47,7 @@ jobs:
             os: windows-latest
             release: true
             shell: msys2 {0}
-            CMAKE_OPTIONS: -G "Unix Makefiles" -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DSTATIC=ON
+            CMAKE_OPTIONS: -G "Unix Makefiles" -DSTATIC=ON
       # complete all jobs
       fail-fast: false
     name: ${{ matrix.name }}
@@ -72,7 +72,7 @@ jobs:
       - name: Install MSYS2
         uses: msys2/setup-msys2@v2
         with:
-          install: make python mingw-w64-x86_64-cmake mingw-w64-x86_64-clang mingw-w64-x86_64-ccache git diffutils
+          install: make python mingw-w64-x86_64-cmake mingw-w64-x86_64-gcc mingw-w64-x86_64-ccache git diffutils
         if: matrix.os == 'windows-latest'
       - name: Cache
         uses: actions/cache@v2

doc/make/msys2.md

@@ -19,7 +19,7 @@ MSYS2 has a package management system, [pacman][pacman], which is used in Arch L
 Here are the commands to install all dependencies needed to compile Lean on your machine.
 
 ```bash
-pacman -S make mingw-w64-x86_64-cmake mingw-w64-x86_64-ccache mingw-w64-x86_64-clang git
+pacman -S make mingw-w64-x86_64-cmake mingw-w64-x86_64-ccache mingw-w64-x86_64-gcc git
 ```
 
 Then follow the [generic build instructions](index.md) in the MSYS2 MinGW shell, using