CI: Attempt to apply cancel-on-progress only on PR:s, not on main

[emilk]

We use cancel-in-progress: true on our GitHub actions, so that new commits to a PR will cancel the currently running CI job, to make the queue shorter and save us CI-dollars.

BUT I just noticed that this also applies to new commits to main, which means we have a trail of canceled CI jobs on main: https://github.com/rerun-io/rerun/commits/main

This is an attempt to fix this. We won't know if it worked until we merge it!

I did make sure that the second commit to this PR canceled the first commit, so at least we haven't made things worse (it seems).

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I've added a line to CHANGELOG.md (if this is a big enough change to warrant it)

[emilk]

When this is approved, I'll merge it, then merge another PR (e.g. #1028) in quick succession, to test if this PR was effectual.

[emilk]

Nope, this didn't work