Bug 1643504 - Upgrade Linux plain builds to Debian buster. r=firefox-build-system-reviewers,rstewart

Differential Revision: https://phabricator.services.mozilla.com/D78425

taskcluster/ci/build/linux.yml

@@ -53,7 +53,7 @@ linux64-plain/opt:
         tier: 2
     worker-type: b-linux
     worker:
-        docker-image: {in-tree: debian9-amd64-build}
+        docker-image: {in-tree: debian10-amd64-build}
         max-run-time: 5400
         env:
             PERFHERDER_EXTRA_OPTIONS: plain
@@ -234,7 +234,7 @@ linux64-plain/debug:
         tier: 2
     worker-type: b-linux
     worker:
-        docker-image: {in-tree: debian9-amd64-build}
+        docker-image: {in-tree: debian10-amd64-build}
         max-run-time: 5400
         env:
             PERFHERDER_EXTRA_OPTIONS: plain