Use github action that caches LFS files when checking out the repo (#647)

Uses https://github.com/nschloe/action-cached-lfs-checkout where we're using git-lfs.

This is a wrapper around `actions/checkout@v3` which uses `actions/cache@v3` to cache the files on LFS to avoid downloading them every time and spare LFS bandwidth.

.github/workflows/validate-lfs.yml

@@ -7,9 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     name: Validate
     steps:
-      - uses: actions/checkout@v3
-        with:
-          lfs: 'true'
+      - uses: nschloe/action-cached-lfs-checkout@v1.2.1
 
       - run: |
           ./tools/git/validate_lfs.sh