ci: pin third-party actions to commit SHAs (#5869)

* ci: pin third-party actions to commit SHAs

Pin third-party GitHub Actions in workflow files to immutable 40-char
commit SHAs per the org security policy. Mutable refs like @v4 can be
silently re-pointed by a compromised upstream; SHAs cannot. Trailing
version comments let Dependabot/Renovate continue to manage updates.

Related to [COR-51](https://linear.app/crewai/issue/COR-51).

* ci: disable persist-credentials in pip-audit checkout

Address CodeRabbit feedback on PR #5869: the pip-audit workflow is
read-only and never needs an authenticated git context, so opt out of
persisting the GITHUB_TOKEN in the local git config per the
actions/checkout security guidance.

.github/workflows/vulnerability-scan.yml

@@ -16,11 +16,13 @@ jobs:
     name: pip-audit
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+        with:
+          persist-credentials: false
 
       - name: Restore global uv cache
         id: cache-restore
-        uses: actions/cache/restore@v4
+        uses: actions/cache/restore@0057852bfaa89a56745cba8c7296529d2fc39830 # v4.3.0
         with:
           path: |
             ~/.cache/uv
@@ -110,14 +112,14 @@ jobs:
 
       - name: Upload pip-audit report
         if: always()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
         with:
           name: pip-audit-report
           path: pip-audit-report.json
 
       - name: Save uv caches
         if: steps.cache-restore.outputs.cache-hit != 'true'
-        uses: actions/cache/save@v4
+        uses: actions/cache/save@0057852bfaa89a56745cba8c7296529d2fc39830 # v4.3.0
         with:
           path: |
             ~/.cache/uv