[gitlab-ci] install yarn packages

Hopefully, --skip-builds option will skip building purescript, which
doesn't work on nixos now...
1 job for 294-dev-test in 1 minute and 42 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #1614
test

00:01:42

 
Name Stage Failure
failed
test Test
[ FAILURE ] Check if 'stack' command is available
Error: Command failed with exit code 2 (ENOENT): stack --allow-different-user --numeric-version
spawn stack ENOENT
at Process.ChildProcess._handle.onexit (internal/child_process.js:269:19)
at onErrorNT (internal/child_process.js:465:16)
at processTicksAndRejections (internal/process/task_queues.js:80:21)
info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.
Cleaning up file based variables
ERROR: Job failed: exit code 1