[gitlab-ci] install yarn packages

Hopefully, --skip-builds option will skip building purescript, which
doesn't work on nixos now...

.gitlab-ci.yml

@@ -29,4 +29,5 @@ test:
     - nix-env -i git
     - nix-env -i nodejs
     - nix-env -i yarn
+    - yarn --skip-builds
     - nix-shell shell.nix --run test-ps