ci: add before_script: tfenv install and cache .tfenv dir

.gitlab-ci.yml

@@ -1,9 +1,10 @@
 default:
   image: ghcr.io/vojtechmares/container-images/tfenv:latest
   cache:
-    key: dot-terraform
+    key: terraform
     paths:
       - .terraform
+      - .tfenv
 
 stages:
   - init
@@ -18,6 +19,8 @@ variables:
 
 init:
   stage: init
+  before_script:
+    - tfenv install
   script:
     - terraform init