chore(workflows): pin GitHub Actions in generate-protos workflow (#4964)

This PR pins the GitHub Actions used in the `generate-protos.yml`
workflow.
 - Pinned `actions/checkout` to `v6.0.2`
 - Pinned `actions/setup-python` to `v6.2.0`
 - Pinned `actions/setup-go` to `v6.2.0`

Author: cuixq

.github/workflows/generate-protos.yml

@@ -27,13 +27,13 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Check out code
-        uses: actions/checkout@v4
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           submodules: recursive
           persist-credentials: false
 
       - name: Set up Python
-        uses: actions/setup-python@v5
+        uses: actions/setup-python@a309ff8b426b58ec0e2a45f0f869d46889d02405 # v6.2.0
         with:
           python-version: '3.13'
 
@@ -46,7 +46,7 @@ jobs:
           cd gcp/api && poetry install
 
       - name: Set up Go
-        uses: actions/setup-go@v5
+        uses: actions/setup-go@7a3fe6cf4cb3a834922a1244abfce67bcef6a0c5 # v6.2.0
         with:
           go-version: '1.26.0'