Workflows: enable CI for PRs

[marcdumais-work]

ATM our two main workflows (1) are triggered by two things:

• nightly cron job
• whenever we merge something to branch master

It would be valuable to have these workflows run for PRs too, except for the publishing part.

(1) https://github.com/eclipse-theia/vscode-builtin-extensions/blob/master/.github/workflows/publish-vsx-latest.yml
and https://github.com/eclipse-theia/vscode-builtin-extensions/blob/master/.github/workflows/publish-vsx-next.yml

[marcdumais-work]

Example on how to make publishing conditional, so it happens only for merges on master:
https://github.com/eclipse-theia/theia/blob/master/.github/workflows/build.yml#L76

I think we want only the first part here, since we want to publish nightly, contrary to upstream