workflows/full-ci: allow manually triggering the workflow (mostly useful for forks)

[0x53A]

First I added this temporarily to my WIP branch with the intention to back it out before merge, but maybe it would make sense to also add this to master.

I wanted to be able to run CI in my fork, and this allows triggering the workflow on a branch manually

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-933

[Bromeon]

Thanks! This can be a nice addition. 🙂

Did you encounter any caveats?

One thing I could imagine is that if the full-ci.yml has any statements that rely on it (unconditionally) being a PR, that would no longer work -- but I'm OK with considering that ill-formed.

If we add this, we should maybe mention it in the book, otherwise it's quite hard to discover:
https://godot-rust.github.io/book/contribute/dev-tools.html

[0x53A]

yeah makes sense I'll add a sentence to the book (https://github.com/godot-rust/book/blob/master/src/contribute/dev-tools.md#continuous-integration).

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I did not encounter any issues; from skimming the workflow definition, it doesn't reference the current PR.

here is one example run: https://github.com/0x53A/gdext/actions/runs/11534881310

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Not related to this PR, but related to CI in general, the workflow Compile + Sync Docs also gets executed on forks if GH Actions are enabled (since the trigger is "push to master").

https://github.com/0x53A/gdext/actions/runs/11534878151/workflow

might make sense to add an if: github.repository == 'godot-rust/gdext'