chore: update Earthfile and GitHub action configuration

- Change Earthfile to import from main branch
- Add caching step in GitHub action for Earthly
- Remove commented import line in Earthfile

.github/actions/env/action.yml

@@ -15,7 +15,7 @@ runs:
     - name: Cache Earthly
       uses: actions/cache@v4
       with:
-        path: /root/.cache
+        path: /tmp/earthly
         key: ${{ runner.os }}-earthly
     - name: Set up QEMU
       uses: docker/setup-qemu-action@v3