Ref actions by commit SHA in shellcheck.yml

It's important to make sure the SHA's are from the original repositories and not forks.

For reference:

https://github.com/actions/checkout/releases/tag/v3.5.2
8e5e7e5ab8

https://github.com/ludeeus/action-shellcheck/releases/tag/2.0.0
00cae500b0


Signed-off-by: Gabriela Gutierrez <gabigutierrez@google.com>

.github/workflows/shellcheck.yml

@@ -8,6 +8,6 @@ jobs:
     name: Shellcheck
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@8e5e7e5ab8b370d6c329ec480221332ada57f0ab # v3.5.2
     - name: Run ShellCheck
-      uses: ludeeus/action-shellcheck@master
+      uses: ludeeus/action-shellcheck@00cae500b08a931fb5698e11e79bfbd38e612a38 # v2.0.0