Currently, the gecko-decision and action tasks maintain a separate cache
for each "project" (Firefox repo) source checkout. In my opinion, this
is not necessary.

Separate caches/source checkouts mean a lower chance for a cache hit:
a lower chance that any given worker will have a source checkout readily
available for the "project" being built. They also mean workers store
more data and files in caches.

By using a shared cache for source checkouts, workers will ideally
only have a single source checkout cache and will be pretty much
guaranteed to have a checkout readily available (the main exception
being the first task on a freshly-provisioned worker).

A downside of a shared cache is that workers may spend more time doing
`hg update` operations. e.g. if the last task was from mozilla-release
and the current task is from mozilla-central, thousands of files will
be updated. However, I believe we will still be better off because
separate caches *increase* the probability for a full checkout, which
involves writing ~150,000 files! So I believe this change will lower
the overall amount of VCS work done by workers while possibly
sacrificing the lowest possible execution time (which is achieved by
minimizing the `hg update` work).

It's worth noting that `hg robustcheckout` (which is used by these
tasks) does the equivalent of an `hg purge --all`. This ensures that all
untracked and ignored files from previous tasks are deleted. So there
should be no contamination from previous tasks.

MozReview-Commit-ID: I5Ng3xFEfou

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extra : rebase_source : db423e5005d1e26d7dfebbbfa28893fc32caef6f