[ci] Avoid duplicate builds on pull requests

Don’t run GitHub Actions on pushing to branches other than master. This
was already the case for the linux-ci workflow.

.github/workflows/msvc-ci.yml

@@ -2,7 +2,9 @@ name: msvc
 
 on:
   push:
+    branches: [ master ]
   pull_request:
+    branches: [ master ]
 
 jobs:
   msvc:

.github/workflows/msys2-ci.yml

@@ -2,7 +2,9 @@ name: msys2
 
 on:
   push:
+    branches: [ master ]
   pull_request:
+    branches: [ master ]
 
 jobs:
   msys2: