Use lock file to prevent concurrent XPI builds

Attempting to run build_xpi concurrently can fail with non-obvious
errors, so it is petter to limit it to one build at time.
Separate building and downloading langack actions

So that we can allow everyone to download the packs, but only few to
build them.
Create extension’s flag if it does not exist

Defaults to disabling the flag for all, even superusers.
Fix Mozilla actions installation instructions
Use “Mozilla” group for all Mozilla actions
Use the same waffle flag for building langpack and downloading tarball
Show some feedback after building the langpack
“langpack” → “language pack” in visible strings
Limit Mozilla actions to firefox and mobile projects
Use translation permissions instead of waffle flag

Here, re-using translation permissions is much easier than duplicating
the list of who are allowed to administrate Mozilla translation projects.
Hide XPI download if the file is not there yet