Fix GitHub Actions not pinned by hash (#178917)

Use explicit pinned dependencies per:
- [The Open Source Security Foundation (OpenSSF) Security
Guide](https://github.com/ossf/scorecard/blob/main/docs/checks.md#pinned-dependencies)

This pull request is created by
[StepSecurity](https://app.stepsecurity.io/securerepo) at the request of
@guidezpl. Please merge the Pull Request to incorporate the requested
changes.

Signed-off-by: StepSecurity Bot <bot@stepsecurity.io>
Co-authored-by: Pierre-Louis <plg@google.com>
Co-authored-by: Zachary Anderson <zanderso@users.noreply.github.com>

.github/workflows/content-aware-hash.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v6
+        uses: actions/checkout@1af3b93b6815bc44a9784bd300feb67ff0d1eeb3 # v6.0.0
 
       - name: Generate Hash
         run: |

.github/workflows/files-changed.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v6
+        uses: actions/checkout@1af3b93b6815bc44a9784bd300feb67ff0d1eeb3 # v6.0.0
 
       - name: Fetch base commit and origin/master
         # Fetch what to compare the commit against
@@ -35,7 +35,7 @@ jobs:
           ./bin/dart run dev/tools/bin/get_files_changed.dart --since="${{ github.event.pull_request.base.sha }}" > changed_files.json
 
       - name: Upload artifact
-        uses: actions/upload-artifact@v5
+        uses: actions/upload-artifact@330a01c490aca151604b8cf639adc76d48f6c5d4 # v5.0.0
         with:
           name: changed-files
           path: changed_files.json

.github/workflows/merge-changelog.yml

@@ -15,7 +15,7 @@ jobs:
 
     steps:
       - name: Setup Repository
-        uses: actions/checkout@v6
+        uses: actions/checkout@1af3b93b6815bc44a9784bd300feb67ff0d1eeb3 # v6.0.0
         with:
           repository: ${{ github.repository }}
           ref: master

.github/workflows/tool-test-general.yml

@@ -22,7 +22,7 @@ jobs:
 
     steps:
       # Note: we must check out the tree for the composite action to be available
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@1af3b93b6815bc44a9784bd300feb67ff0d1eeb3 # v6.0.0
       - uses: ./.github/actions/composite-flutter-setup
 
       - name: Tool Test