Separate build and testing phases in GitLab Workflow extension

Right now the build phase of the Workflow extension does two things at once:

1. esbuild: Build the extension into a single file with all the assets
2. tsc: Compiles the source code and tests into the dist-desktop folder, also producing package.json

This is not ideal because the build step also produces a new package.json file which gets replaces by the tsc compilation command.

We should decouple these two commands and do only type checking for the build phase and extract source compilation into the integration tests preparation phase.