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

@@ -12,6 +12,11 @@ runs:
         github-token: ${{ inputs.token }}
         version: "latest"
         use-cache: true
+    - name: Cache Earthly
+      uses: actions/cache@v4
+      with:
+        path: /root/.cache
+        key: ${{ runner.os }}-earthly
     - name: Set up QEMU
       uses: docker/setup-qemu-action@v3
     - name: Set up Docker Buildx
@@ -25,5 +30,4 @@ runs:
         else
           branch="${GITHUB_REF##*/}"
         fi
-        git checkout -b "$branch" || true
-
+        git checkout -b "$branch" || true

Earthfile

@@ -1,6 +1,7 @@
 VERSION 0.8
 
-IMPORT github.com/formancehq/earthly:tags/v0.16.2 AS core
+IMPORT github.com/formancehq/earthly:main AS core
+#IMPORT github.com/formancehq/earthly:tags/v0.16.2 AS core
 
 FROM core+base-image