[CI] enable post-commit as a workflow action. (#7289)

It'd be nice to be able to trigger post-commit checks, especially when
we aren't able to test locally. This PR makes post-commit as a
dispatchable workflow.

Co-authored-by: Pavel Chupin <[email protected]>

Author: cperkinsintel

.github/workflows/sycl_post_commit.yml

@@ -13,7 +13,7 @@ on:
     - .github/workflows/sycl_linux_build_and_test.yml
     - .github/workflows/sycl_windows_build_and_test.yml
     - .github/workflows/sycl_macos_build_and_test.yml
-
+  workflow_dispatch:
 
 jobs:
   # This job generates matrix of tests for LLVM Test Suite