Specify tox environment with an env var

This will make it easier to override.

.circleci/config.yml

@@ -38,6 +38,8 @@ jobs:
       # Tell the C runtime things about character encoding (mainly to do with
       # filenames and argv).
       - LANG: "en_US.UTF-8"
+      # Select a tox environment to run for this job.
+      - TAHOE_LAFS_TOX_ENVIRONMENT: "coverage"
 
     steps:
       - run:
@@ -95,7 +97,7 @@ jobs:
             # Also run with /tmp as a workdir because the non-root user won't
             # be able to create the tox working filesystem state in the source
             # checkout because it is owned by root.
-            sudo --set-home -u nobody /tmp/tests/bin/tox -c /tmp/project/tox.ini --workdir /tmp -e coverage
+            sudo --set-home -u nobody /tmp/tests/bin/tox -c /tmp/project/tox.ini --workdir /tmp -e ${TAHOE_LAFS_TOX_ENVIRONMENT}
 
       - store_artifacts: &STORE_ARTIFACTS
           # Despite passing --workdir /tmp to tox above, it still runs trial