Introduce `hg purge` options
Heptapod Runner performs an unconditional hg purge --all
, following to GitLab Runner's git clean
that was also unconditional.
Since then, upstream introduced the GIT_CLEAN_FLAGS
environment variable to give job authors finer control on the git clean
call.
We should introduce a HG_PURGE_OPTIONS
or HG_CLEAN_OPTIONS
environment variable as well