Add marker to skip CI invocation

.github/workflows/master.yml

@@ -15,6 +15,7 @@ jobs:
           - build_type: 'generic'
             build_cc: gcc-10
             build_cflags: '-O3 -Wno-error=maybe-uninitialized'
+    if: "!contains(github.event.head_commit.message, 'skip ci')"
     steps:
     - name: Checkout Sources
       uses: actions/checkout@v2