Add pull request trigger to the build workflow

[SunSerega]

I recently figured out with munix that the push trigger created GitHub Action in the repo that had the push.
So if the commit is pushed to my fork, no Action is created in this repo and so my PR doesn't have the "Checks successful" badge:
https://discord.com/channels/1078696971088433153/1078841771771056280/1215255763518230528

Btw, this is already fixed in the latest version of the template.
And I see you somehow already updated the template before, right?
Mind sharing your process for this kind of update? I've set up this for myself, but now I'm thinking if there was an easier route...

[SunSerega]

Ah, you can also need to go to:
https://github.com/Falki-git/MicroEngineer/settings/actions
And set Fork pull request workflows from outside collaborators to Require approval for first-time contributors who are new to GitHub.
It's on Require approval for first-time contributors by default just in case, but allowing new contributors to run build.yml on your repo can't really be a vulnerability...