Reduce duplicate GitHub Action runs

[danyeaw]

Currently, when pushing a branch and opening a PR, GitHub Actions runs two complete builds. This PR tries to reduce duplicate runs by only running the full build when:

1. A PR is opened against the main branch
2. Push to the main branch
3. On release
4. On manually trigger a build with workflow dispatch from the actions menu

The tradeoff of this will be that if you want to run GitHub Actions, you'll need to open a draft PR or manually start it from the Actions tab, since pushing to a branch will not start a run.

I also added logic to cancel running previous workflows using concurrency.

PR Checklist

Please check if your PR fulfills the following requirements:

• I have read, and I am following the Contributor guide
• I have read, and I understand the GNOME Code of Conduct

PR Type

What kind of change does this PR introduce?

• Bug fix
• Feature
• Chore (refactoring, formatting, local variables, other cleanup)
• Documentation content changes

What is the current behavior?

2 full builds for each push of a PR

Issue Number: N/A

What is the new behavior?

Avoid duplicate builds under most circumstances

Does this PR introduce a breaking change?

• Yes
• No

Other information

[amolenaar]

I like the ability to run a pipeline before I create a PR. But I agree that building for both pull and PR is overhead we can do without.

How does it work for forks? Should they open a PR in order to build something?

[danyeaw]

I like the ability to run a pipeline before I create a PR. But I agree that building for both pull and PR is overhead we can do without.

I think the workflow dispatch would help fill in this use case by manually running a workflow by selecting the branch you push.

How does it work for forks? Should they open a PR in order to build something?

Yup, create a PR.

[amolenaar]

I like the ability to run a pipeline before I create a PR. But I agree that building for both pull and PR is overhead we can do without.

I think the workflow dispatch would help fill in this use case by manually running a workflow by selecting the branch you push.

Fair enough. That's an option indeed.

How does it work for forks? Should they open a PR in order to build something?

Yup, create a PR.

This implies they cannot check if all builds, unless a build id triggered manually. Which is fine, I suppose.