Allow CI to be skipped if the commit message starts with [skip ci].

Author: bcoconni

.github/workflows/build_test.yml

@@ -9,6 +9,7 @@ defaults:
 
 jobs:
   test:
+    if: ${{ !startsWith(github.event.head_commit.message, '[skip ci]') || github.event_name == 'pull_request' }}
     name: Julia ${{ matrix.version }} - ${{ matrix.os }} - ${{ matrix.arch }}
     runs-on: ${{ matrix.os }}
     strategy: