Don't run GitHub CI action when other CI scripts change

Avoid wasting resources when only the scripts used with AppVeyor or
Travis CI change.

.github/workflows/ci.yml

@@ -6,6 +6,9 @@ on:
     branches:
       - master
     paths-ignore:
+      - 'build/tools/appveyor*.bat'
+      - 'build/tools/before_install.sh'
+      - 'build/tools/travis-ci.sh'
       - 'distrib/**'
       - 'docs/**'
       - 'interface/**'
@@ -15,6 +18,9 @@ on:
     branches:
       - master
     paths-ignore:
+      - 'build/tools/appveyor*.bat'
+      - 'build/tools/before_install.sh'
+      - 'build/tools/travis-ci.sh'
       - 'distrib/**'
       - 'docs/**'
       - 'interface/**'