workflows/test: factor out steps & refactoring

.github/actions/build-foo-cygwin/action.yml

@@ -0,0 +1,20 @@
+name: Build foo.exe
+description: Build foo.exe (on Cygwin)
+inputs:
+  platform:
+    description: Target platform
+    required: true
+runs:
+  using: composite
+  steps:
+    - run: |
+        IFS= read -r src_dir <<'EOF'
+        ${{ github.workspace }}
+        EOF
+        echo "$src_dir"
+        src_dir="$( cygpath -ua "$src_dir" )"
+        cd -- "$src_dir"
+        arch=
+        [ '${{ matrix.platform }}' == 'x86' ] && arch=-m32
+        clang++ $arch -std=c++14 -o foo.exe foo.cpp
+      shell: C:\tools\cygwin\bin\bash.exe --login --norc -eo pipefail -o igncr '{0}'