Bug 1290516 - Remove {{project}} from cache path for decision and act…

…ion tasks; r=dustin

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

--HG--
extra : rebase_source : db423e5005d1e26d7dfebbbfa28893fc32caef6f

.taskcluster.yml

@@ -87,7 +87,7 @@ tasks:
 
         cache:
           level-{{level}}-hg-shared: /home/worker/hg-shared
-          level-{{level}}-{{project}}-gecko-decision: /home/worker/workspace
+          level-{{level}}-gecko-decision: /home/worker/workspace
 
         features:
           taskclusterProxy: true

taskcluster/taskgraph/action.yml

@@ -37,7 +37,7 @@ payload:
 
   cache:
     level-{{level}}-hg-shared: /home/worker/hg-shared
-    level-{{level}}-{{project}}-gecko-decision: /home/worker/workspace
+    level-{{level}}-gecko-decision: /home/worker/workspace
 
   features:
     taskclusterProxy: true