[CI] Restrict workflows to intel/llvm repo (#3558)

Author: Alexander Batashev

.github/workflows/linux_post_commit.yml

@@ -7,6 +7,7 @@ on:
 jobs:
    check:
      runs-on: ubuntu-18.04
+     if: github.repository == 'intel/llvm'
      strategy:
        fail-fast: false
        matrix:

.github/workflows/sync-main.yml

@@ -6,6 +6,7 @@ on:
 jobs:
   sync:
     runs-on: ubuntu-latest
+    if: github.repository == 'intel/llvm'
     steps:
       - uses: actions/checkout@v2
         with: