Some jobs might fail now after latest update since now `godot-cpp-directory` is mandatory param

[Ughuuu]

This PR: #47 introduced a new mandatory param, godot-cpp-directory, which if not set or if the directory godot-cpp doesn't exist, existing build jobs will fail unless they update. We should not break compatibility, either we should make it optional(and if it not exists, jobs should not fail). Right now it fails with:
https://github.com/libriscv/godot-sandbox/actions/runs/11711734886/job/32620908126

Error: An error occurred trying to start process '/usr/bin/sh' with working directory '/home/runner/work/godot-sandbox/godot-sandbox/godot-cpp/'. No such file or directory

[Ivorforce]

Hi! I'm sorry about the failure, this came from my recent contribution (d100b72).

The runner now assumes that godot-cpp exists in the ./godot-cpp directory, and will fail if it can't find it. You can pass the godot-cpp-directory parameter to point it to the right folder instead:

godot-cpp-template/.github/actions/build/action.yml

Lines 26 to 28 in d100b72

	godot-cpp-directory:
	default: 'godot-cpp/'
	description: Location of godot-cpp in the repository. Must not contain relative path signifiers (. or ..). Must be a transparent path part (empty or 'path/to/directory/', ending in a slash).

(This assumption previously existed too, but the runner had a bug, and didn't run the command in the right directory, so it "incorrectly succeeded", in a way)

For the future, I'd advise to base runners off a tag or git hash instead of a branch, to avoid branches that fail without a manual update.