Pin GitHub Actions to commit SHAs

.github/workflows/validate-frontmatter.yml

@@ -17,7 +17,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - uses: oven-sh/setup-bun@v2
+      - uses: oven-sh/setup-bun@0c5077e51419868618aeaa5fe8019c62421857d6 # v2.2.0 (sha-pinned)
 
       - name: Install dependencies
         run: cd .github/scripts && bun install yaml