Expands on #3971 to do something useful even before the PR enters the
queue:
If stage0 changes are detected in the PR, set the changes-stage0 label
(which
has a tooltip to explain what this entail), and also remove the label if
it no
longer applies.
This Github action automatically updates `stage0` on `master` if
`src/stdlib_flags.h` and `stage0/src/stdlib_flags.h`
are out of sync there.
It bypasses the merge queue to be quick, this way, an out-of-date stage0
on on
master should only exist for a few minutes.
Needs access to a _deploy SSH key_ with write permission.
