Pin remaining GitHub Actions (#733)

## Summary

- Follow-up to #725 — pins remaining action references that were missed

## Test plan

- [x] Verified all action references pass the pin scan

This pull request was AI-assisted by Isaac.

Author: pietern

.github/workflows/external-message.yml

@@ -21,7 +21,7 @@ jobs:
       pull-requests: write
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
 
       - name: Delete old comments
         env:

.github/workflows/integration-tests.yml

@@ -41,7 +41,7 @@ jobs:
     environment: "test-trigger-is"
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@f43a0e5ff2bd294095638e18286ca9a3d1956744 # v3.6.0
 
       - name: Generate GitHub App Token for Workflow Trigger
         id: generate-token

.github/workflows/next-changelog.yml

@@ -13,7 +13,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v3
+        uses: actions/checkout@f43a0e5ff2bd294095638e18286ca9a3d1956744 # v3.6.0
 
       - name: Fetch list of changed files
         id: changed-files