chore(deps): bump chainguard-dev/actions (#5929)

Bumps [chainguard-dev/actions](https://github.com/chainguard-dev/actions) from 1.6.19 to 1.6.21.
- [Release notes](https://github.com/chainguard-dev/actions/releases)
- [Commits](chainguard-dev/actions@v1.6.19...v1.6.21)

---
updated-dependencies:
- dependency-name: chainguard-dev/actions
  dependency-version: 1.6.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/actions/build-custom-melange-package/action.yml

@@ -46,7 +46,7 @@ runs:
         export GIT_TAG=${{ inputs.git-tag }}
         envsubst '${GIT_TAG}' < ${{ inputs.context }}/melange.yaml.tmpl > ${{ inputs.context }}/melange.yaml
 
-    - uses: chainguard-dev/actions/melange-build@v1.6.19
+    - uses: chainguard-dev/actions/melange-build@v1.6.21
       with:
         config: ${{ inputs.context }}/melange.yaml
         archs: ${{ inputs.arch }}