[ci] run Appveyor checks on PRs targeting release/ branches (#5528)

.appveyor.yml

@@ -6,7 +6,7 @@ configuration:  # a trick to construct a build matrix with multiple Python versi
   - '3.7'
 
 # only build pull requests and
-# commits to 'master'
+# commits to 'master' or any branch starting with 'release'
 branches:
   only:
     - master