Bug 1967322 - don't share test variant's environment across all tasks using it. r=ahal,taskgraph-reviewers

Differential Revision: https://phabricator.services.mozilla.com/D250253

taskcluster/gecko_taskgraph/transforms/job/mozharness_test.py

@@ -172,6 +172,8 @@ def mozharness_test_on_docker(config, job, taskdesc):
     # Set MOZ_ENABLE_WAYLAND env variables to enable Wayland backend.
     if "wayland" in job["label"]:
         env["MOZ_ENABLE_WAYLAND"] = "1"
+    else:
+        assert "MOZ_ENABLE_WAYLAND" not in env
 
     if mozharness.get("mochitest-flavor"):
         env["MOCHITEST_FLAVOR"] = mozharness["mochitest-flavor"]

taskcluster/gecko_taskgraph/transforms/test/variant.py

@@ -114,7 +114,7 @@ def split_variants(config, tasks):
             variant=name,
         )
 
-        return merge(task, variant.get("merge", {}))
+        return merge(task, deepcopy(variant.get("merge", {})))
 
     expired_variants = find_expired_variants(TEST_VARIANTS)
     for task in tasks: