CI: Use modern --import syntax in GHA

[Repiteo]

• Related: Fix Editor crash during first scan in headless import mode. #108992

Replaces the legacy --editor --quit syntax with --import when running the Godot Test Project action. Could potentially also apply --headless, but its exclusion might be by design?

[AThousandShips]

LGTM

[Calinou]

Could potentially also apply --headless, but its exclusion might be by design?

Yes, it's likely by design for two reasons:

• We want to test the editor's graphical codepaths (CI has llvmpipe configured, so it works without a physical GPU in place).
• There are some bugs in --headless import, e.g. when using global/per-instance shader parameters. This is what GitHub Actions: Test project exporting on CI #109146 aims to track.