run github actions only once on PR

[knoxfighter]

Prevent running the github actions twice, when we create a PR from a branch on this repo.

[knoxfighter]

This has only small potential to work!
A call to 'actions/cancel-action' needs to have actions:write permission, which it hasn't, when it is called from a PR.

It could work, when it is run from the this repo (which would be fine, cause this code will only run from this repo).

Another point: Get the name of the resulting repo (in case of this PR it would be knoxfighter)

[knoxfighter]

It seems like it works, with a custom action i wrote on my user.

Current behaviour will instantly cancel the PR if the base branch is on this repo (hardcoded).

Behavior can also be seen in PR #248 with cancelled build https://github.com/OpenFactorioServerManager/factorio-server-manager/actions/runs/522704849.

@mroote If you're fine with that behaviour, i will move the action also to this org and change it in the workflow.

[mroote]

Wow that's cool nice one! This action might be useful to other people too if you want to make it more generalized. Looks good to merge for this repo though.