Bug 1274655 - Set MOZ_SOURCE_REPO in mozharness to avoid variance from shared repos; r=jlund

Recent changes to mozharness in bug 1270317 started using pooled shared
storage for Mercurial repos. This means the "default" path in Mercurial
repos is variable depending on which repo was the first to be built on
a machine.

By default, the Firefox build system resolves the source repository
from `hg paths default`. This is now incorrect default behavior in
automation.

We fix the regression by setting MOZ_SOURCE_REPO in the environment to
path to the repository that mozharness is currently building.

MozReview-Commit-ID: 34IPf7PJfuA

--HG--
extra : rebase_source : d0d717847f1e52444ecd53ddf16716bad42809eb
extra : source : a2a4c06d9736850782d8cc52802e0207ca1bf27a

testing/mozharness/mozharness/mozilla/building/buildbase.py

@@ -849,6 +849,13 @@ or run without that action (ie: --no-{action})"
         # first grab the buildid
         env['MOZ_BUILD_DATE'] = self.query_buildid()
 
+        # Set the source repository to what we're building from since
+        # the default is to query `hg paths` which isn't reliable with pooled
+        # storage
+        repo_path = self._query_repo()
+        assert repo_path
+        env['MOZ_SOURCE_REPO'] = repo_path
+
         if self.query_is_nightly() or self.query_is_nightly_promotion():
             if self.query_is_nightly():
                 # nightly promotion needs to set update_channel but not do all the 'IS_NIGHTLY'