Add T-linux-wpt label

[Loirooriol]

To run WPT both in 2013 and 2020.

Right now I either need to

• Use T-full, which tests all linux, windows and mac, and includes unit tests. Sometimes I don't care about that.
• Use both T-linux-wpt-2013 and T-linux-wpt-2020, which triggers 2 different actions, and compiles twice.

[sagudev]

#30694 already has logic for it, only GH label is missing.

[mrobinson]

To run WPT both in 2013 and 2020.
* Use both T-linux-wpt-2013 and T-linux-wpt-2020, which triggers 2 different actions, and compiles twice.

This doesn't actually compile twice depending on timing of the GitHub Actions. I've noticed that most of the time, this only does one build. I think we can make this work even more consistently with https://docs.github.com/en/actions/using-jobs/using-concurrency. This should allow us to make sure that labels applied at the same time are combined into the same job.

[mrobinson]

The benefit of using concurrency is that it should allow any combination of jobs without having to add a bunch of extra labels, if we can get it working properly. See #30756.

[sagudev]

Merging of wpt runs was done in #31347. Also try-parser already supports linux-wpt and wpt so it's just matter of adding new tags to github if we really needed to:

servo/python/servo/try_parser.py

Lines 84 to 85 in 06aeeeb

	elif s in ["wpt", "linux-wpt"]:
	return JobConfig("Linux WPT", Workflow.LINUX, unit_tests=True, wpt_layout=Layout.all())