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/build-uv-cache.yml

@@ -23,7 +23,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
 
       - name: Install uv
         uses: astral-sh/setup-uv@d0cc045d04ccac9d8b7881df0226f9e82c39688e # v6
@@ -39,7 +39,7 @@ jobs:
           echo "Cache populated successfully"
 
       - name: Save uv caches
-        uses: actions/cache/save@v4
+        uses: actions/cache/save@0057852bfaa89a56745cba8c7296529d2fc39830 # v4.3.0
         with:
           path: |
             ~/.cache/uv