ci: verify run-task and fetch-content still support Python 3.8

[jcristau]

Consumers can run these scripts with system Python versions older than taskgraph's own minimum. Add a check task that statically scans them with vermin and smoke tests them under a real 3.8 interpreter.

[jcristau]

I think we used to run ok with 3.6 until #1045, and I'd rather these bumps were done on purpose rather than by accident since otherwise we're silently breaking backwards compat for consumers.

[jcristau]

I think we used to run ok with 3.6 until #1045

Actually #1039 did it first, with its use of typing.Literal.
And #594 before that, by using subprocess.run(..., capture_output=True, text=True), required 3.7 but only in the "install requirements with uv" case, so it never came up.