CI: Add a GitHub Action to build on Ubuntu 20.04 LTS

Also clean up MSYS2 action.

.github/workflows/build_win.yml

@@ -2,9 +2,11 @@ name: build-win
 
 on:
   push:
-      branches: [ master ]
   pull_request:
 
+env:
+  BUILD_TYPE: Release
+
 jobs:
   build:
     runs-on: windows-latest
@@ -17,23 +19,22 @@ jobs:
           #{ msystem: MINGW32, arch: i686   }
         ]
     steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
 
-      - name: Checkout
-        uses: actions/checkout@v2
-        with:
-          fetch-depth: 0
+    - name: Setup
+      uses: msys2/setup-msys2@v2
+      with:
+        msystem: ${{ matrix.msystem }}
+        install: mingw-w64-${{ matrix.arch }}-gcc mingw-w64-${{ matrix.arch }}-cmake mingw-w64-${{ matrix.arch }}-make mingw-w64-${{ matrix.arch }}-capstone mingw-w64-${{ matrix.arch }}-glfw mingw-w64-${{ matrix.arch }}-glm mingw-w64-${{ matrix.arch }}-file mingw-w64-${{ matrix.arch }}-llvm mingw-w64-${{ matrix.arch }}-nlohmann-json mingw-w64-${{ matrix.arch }}-openssl mingw-w64-${{ matrix.arch }}-polly mingw-w64-${{ matrix.arch }}-python
+        update: true
 
-      - name: Build
-        uses: msys2/setup-msys2@v2
-        with:
-          msystem: ${{ matrix.msystem }}
-          install: mingw-w64-${{ matrix.arch }}-gcc mingw-w64-${{ matrix.arch }}-cmake mingw-w64-${{ matrix.arch }}-make mingw-w64-${{ matrix.arch }}-capstone mingw-w64-${{ matrix.arch }}-glfw mingw-w64-${{ matrix.arch }}-glm mingw-w64-${{ matrix.arch }}-file mingw-w64-${{ matrix.arch }}-llvm mingw-w64-${{ matrix.arch }}-nlohmann-json mingw-w64-${{ matrix.arch }}-openssl mingw-w64-${{ matrix.arch }}-polly mingw-w64-${{ matrix.arch }}-python
-          update: true
-
-      - name: Build
-        shell: msys2 {0}
-        run: |
-          mkdir build
-          cd build
-          cmake -G "MinGW Makefiles" ..
-          mingw32-make -j 4
+    - name: Build
+      shell: msys2 {0}
+      run: |
+        mkdir build
+        cd build
+        cmake -G "MinGW Makefiles" ..
+        mingw32-make -j 4