gitlab-ci: stop caching ~/.vagrant.d

The main reason to cache vagrant.d is to avoid downloading boxes. This
is implemented on the gitlab-runner itself by bind-mounting only the
boxes directory (.vagrant.d/boxes) from the host instead.

.gitlab-ci.yml

@@ -10,28 +10,18 @@ variables:
 run_base_test:
   stage: test
   script:
-    - mkdir -p .vagrant.d && ln -s $PWD/.vagrant.d $HOME/.vagrant.d
     - "cd test && timeout 1h with-ssh-key ./run-test.sh --docker ${APT_PROXY_ADDR:+--apt-proxy $APT_PROXY_ADDR} base"
   tags: [ai3]
   except:
     - schedules
-  cache:
-    key: buster
-    paths:
-      - .vagrant.d
 
 run_full_test:
   stage: test
   script:
-    - mkdir -p .vagrant.d && ln -s $PWD/.vagrant.d $HOME/.vagrant.d
     - "cd test && timeout 1h with-ssh-key ./run-test.sh --save-logs ${CI_PROJECT_DIR}/test-logs --docker --wait-time 300 ${APT_PROXY_ADDR:+--apt-proxy $APT_PROXY_ADDR} full"
   tags: [ai3]
   only:
     - schedules
-  cache:
-    key: buster
-    paths:
-      - .vagrant.d
   artifacts:
     when: on_failure
     expire_in: 1 week