Update triage workflow

[leofang]

Right now we add the triage label whenever an issue is opened or reopened

cuda-python/.github/workflows/triagelabel.yml

Lines 2 to 6 in e1e3325

	on:
	issues:
	types:
	- reopened
	- opened

but it could be cumbersome if an issue is created by a team member who already knows the issue categories and can add the label at issue creation time. We need a slightly more sophisticated check, e.g. if an issue already has nonzero labels, before adding the triage label. (In order to ensure this logic works, we should also disable external contributors' ability to tag any label themselves.)

[leofang]

@jollylili is this something you can help with? 🙂

[leofang]

Consolidating this with #110.