Remove needless cd from gitlab-ci.

It seems that the workdir for gitlab is already inside the cloned repo,
so `cd $EXPORT_NAME` will fail.

Here's a build that prints `pwd` to demonstrate:
https://gitlab.com/rcorre/weekly-game-jam-204/-/jobs/1318895344

Here's a pipeline that passed after removing `cd`:
- https://gitlab.com/rcorre/weekly-game-jam-204/-/pipelines/315008027
- 1af90b1fa2.

.gitlab-ci.yml

@@ -1,5 +1,8 @@
 image: barichello/godot-ci:3.2.3
 
+# NOTE: the `cd` command in each build stage is not necessary if your
+# project.godot is at the repo root
+
 # Cache imported assets between runs
 cache:
   key: import-assets