Bug 1574668 - switch to community TC deployment (#20039)

Within the new "Community" deployment of Taskcluster, the worker pools
(provisionerId/workerType) are different -- WPT has a dedicated worker
pool all to itself.  This uses the new worker pool names in the new
deployment.  Once things are migrated to the new deployment, the logic
here can be removed and replaced with simple strings.
diff --git a/.taskcluster.yml b/.taskcluster.yml
index 27479fb..6de1e28 100644
--- a/.taskcluster.yml
+++ b/.taskcluster.yml
@@ -4,6 +4,19 @@
 tasks:
   $let:
     event_str: {$json: {$eval: event}}
+    provisionerId:
+      $if: 'taskcluster_root_url == "https://taskcluster.net"'
+      then: aws-provisioner-v1
+      else: proj-wpt
+    workerType:
+      $if: 'taskcluster_root_url == "https://taskcluster.net"'
+      then:
+        $if: event.repository.full_name == 'web-platform-tests/wpt'
+        then:
+          wpt-docker-worker
+        else:
+          github-worker
+      else: ci
   in:
     $flattenDeep:
       - $if: tasks_for == "github-push"
@@ -52,13 +65,8 @@
               taskGroupId: {$eval: 'as_slugid("task group")'}
               created: {$fromNow: ''}
               deadline: {$fromNow: '24 hours'}
-              provisionerId: aws-provisioner-v1
-              workerType:
-                $if: event.repository.full_name == 'web-platform-tests/wpt'
-                then:
-                  wpt-docker-worker
-                else:
-                  github-worker
+              provisionerId: ${provisionerId}
+              workerType: ${workerType}
               metadata:
                 name: wpt-${browser.name}-${browser.channel}-${chunk[0]}-${chunk[1]}
                 description: >-
@@ -155,13 +163,8 @@
                 taskGroupId: {$eval: 'as_slugid("task group")'}
                 created: {$fromNow: ''}
                 deadline: {$fromNow: '24 hours'}
-                provisionerId: aws-provisioner-v1
-                workerType:
-                  $if: event.repository.full_name == 'web-platform-tests/wpt'
-                  then:
-                    wpt-docker-worker
-                  else:
-                    github-worker
+                provisionerId: ${provisionerId}
+                workerType: ${workerType}
                 metadata:
                   name: ${operation.name}
                   description: ${operation.description}
@@ -321,13 +324,8 @@
                 taskGroupId: {$eval: 'as_slugid("task group")'}
                 created: {$fromNow: ''}
                 deadline: {$fromNow: '24 hours'}
-                provisionerId: aws-provisioner-v1
-                workerType:
-                  $if: event.repository.full_name == 'web-platform-tests/wpt'
-                  then:
-                    wpt-docker-worker
-                  else:
-                    github-worker
+                provisionerId: ${provisionerId}
+                workerType: ${workerType}
                 metadata:
                   name: ${operation.name}
                   description: ${operation.description}