Bug 1694784 - Run the linux64 PGO profiling step on the ubuntu1804-test image. r=firefox-build-system-reviewers,sheehan,mhentges

Initially, they were using a debian7-amd64-build image, when linux64 were
the only 3-step PGO builds. There wasn't really a profound reason not to
use another image. Then later bug 632954 and bug 1561147 added android
and linux32, using desktop1604-test images, the test image back then,
with no rationale given as to why using different image than builds, but
there's also no reason not to use the test images.

As we're going to remove things from the build images that will probably
prevent the profiling step to run, we might as well use the test image
(now ubuntu1804-test) everywhere.

Differential Revision: https://phabricator.services.mozilla.com/D106356

taskcluster/ci/generate-profile/kind.yml

@@ -70,7 +70,7 @@ jobs:
             env:
                 NEED_XVFB: "true"
             max-run-time: 1200
-            docker-image: {in-tree: debian10-amd64-build}
+            docker-image: {in-tree: ubuntu1804-test}
             artifacts:
                 - type: directory
                   name: public/build