Bug 1602863 - update references to linux64 in awsy.yml to linux1804-64 r=Marian

Changes:

Due to changes I made in D58204 and D58464, `awsy` was migrated to run under the `linux1804-64` label. This was fine except `awsy.yml` used `linux64` to set scheduling.

This patch will update the references to `linux1804-64` and restore proper scheduling of tasks.

Differential Revision: https://phabricator.services.mozilla.com/D59017

--HG--
extra : moz-landing-system : lando

taskcluster/ci/test/awsy.yml

@@ -28,7 +28,7 @@ job-defaults:
             default: default
     fission-tier:
         by-test-platform:
-            linux64(?!-shippable).*: 3
+            linux1804-64(?!-shippable).*: 3
             windows10-64(-qr)?/opt: 3
             default: 2
     run-on-projects:
@@ -38,7 +38,7 @@ job-defaults:
             default: ['try', 'integration', 'mozilla-central', 'mozilla-beta', 'mozilla-release']
     fission-run-on-projects:
         by-test-platform:
-            linux64-shippable-qr/opt: ['mozilla-central']
+            linux1804-64-shippable-qr/opt: ['mozilla-central']
             windows10-64-shippable-qr/opt: ['mozilla-central']
             default: []