ci: pin vouch action version (#3160)

Pins vouch actions to `c6d80ead49839655b61b422700b7a3bc9d0804a9`
(v1.4.2) in favor of security practices. We were previously using the
`@main` tag.

Also removes the checkout steps as they're not needed in these
workflows.

.github/workflows/vouch-check-pr.yml

@@ -13,8 +13,7 @@ jobs:
   check-pr:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
-      - uses: mitchellh/vouch/action/check-pr@main
+      - uses: mitchellh/vouch/action/check-pr@c6d80ead49839655b61b422700b7a3bc9d0804a9 # v1.4.2
         with:
           pr-number: ${{ github.event.pull_request.number }}
           auto-close: true

.github/workflows/vouch-manage-by-issue.yml

@@ -16,8 +16,7 @@ jobs:
       contains(github.event.comment.body, 'denounce') ||
       contains(github.event.comment.body, 'unvouch')
     steps:
-      - uses: actions/checkout@v4
-      - uses: mitchellh/vouch/action/manage-by-issue@main
+      - uses: mitchellh/vouch/action/manage-by-issue@c6d80ead49839655b61b422700b7a3bc9d0804a9 # v1.4.2
         with:
           comment-id: ${{ github.event.comment.id }}
           issue-id: ${{ github.event.issue.number }}