test: only run actions on master

[nobkd]

Only run tests on commits / PRs to branch master or on manual dispatch from actions tab.
This prevents unnecessary runs and therefore saves action time usage.

Also, people typically don't do commits to the default branch of a fork, which also prevents workflows from running in their forked repo.
And if they have to, they can still run the workflow manually.

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Just adding the billable time as a note: https://docs.github.com/en/billing/managing-billing-for-github-actions/about-billing-for-github-actions#included-storage-and-minutes

GitHub Free for organizations: 2,000 minutes (per month)

Operating system	Minute multiplier
Linux	1
Windows	2
macOS	10

I mean, this won't really ever get hit. If we think of 30 days in a month:
$\frac{2000}{30}=66.\overline{6}$ minutes per day... which is won't be reached realistically. We take about 5 seconds for a run with this workflow. Even if there are more workflows...

[fritx]

This prevents unnecessary runs and therefore saves action time usage.
GitHub Free for organizations: 2,000 minutes (per month)

Oops.. My bad. I didn't know it has a quota.. I thought it was totally free.

And if they have to, they can still run the workflow manually.

Makes sense. But an automatically run and a "pass or fail" logo display on a commit or a PR is quite useful, especially when maintainers doing code-review and considering is it safe to merge.

[nobkd]

On a commit to master or a PR to master the test should be run. If new commits are added to the pr, the workflow should run again. (See nobkd#1)

Therefore I think, that's alright.