Bug 1247168 - Actually use workspace cache in decision task; r=dustin

The decision task configures a /home/worker/workspace cache. However,
the command we run in the container references a "workspace" relative
path. From logs in automation, it appears that PWD during execution
is "/" because "workspace" is being resolved to "/workspace." The
net result of this is we appear to be performing a VCS clone+checkout
on every single task.

This commit fixes the paths so our workspace cache is actually used.

MozReview-Commit-ID: Kj6REep5bSs

--HG--
extra : rebase_source : 8cd8be43dfd34f2970b47721c3da8e3957a8bfed
extra : histedit_source : b81dce523a88e44eb3fa8b1a68840066edca382d

.taskcluster.yml

@@ -89,8 +89,8 @@ tasks:
           - -cx
           - >
             mkdir -p /home/worker/artifacts &&
-            checkout-gecko workspace &&
-            cd workspace/gecko &&
+            checkout-gecko /home/worker/workspace &&
+            cd /home/worker/workspace/gecko &&
             ln -s /home/worker/artifacts artifacts &&
             ./mach taskgraph decision
             --pushlog-id='{{pushlog_id}}'

taskcluster/taskgraph/action.yml

@@ -54,8 +54,8 @@ payload:
     - -cx
     - >
       mkdir -p /home/worker/artifacts &&
-      checkout-gecko workspace &&
-      cd workspace/gecko &&
+      checkout-gecko /home/worker/workspace &&
+      cd /home/worker/workspace/gecko &&
       ln -s /home/worker/artifacts artifacts &&
       ./mach taskgraph action-task
       --decision-id='{{decision_task_id}}'