workflows: be more conservative about launching msys

we don't want to lanuch it if we're only modifying a different workflow
template.

.github/workflows/msys2.yml

@@ -2,7 +2,15 @@ name: msys2
 
 on:
   push:
+    paths:
+      - "mesonbuild/**"
+      - "test cases/**"
+      - ".github/workflows/msys.yml"
   pull_request:
+    paths:
+      - "mesonbuild/**"
+      - "test cases/**"
+      - ".github/workflows/msys.yml"
 
 jobs:
   test:
@@ -83,4 +91,4 @@ jobs:
       - uses: actions/upload-artifact@v2
         with:
           name: ${{ matrix.NAME }}
-          path: meson-test-run.*
+          path: meson-test-run.*