fix(tox.ini): set build dir to match where CI jobs expect it to be

Although we write code coverage information to build/coverage.xml, our
CI jobs expect to find it at buils/py3/coverage.xml. Update tox.ini to
match.

tox.ini

@@ -25,7 +25,7 @@ skipsdist=True
 # Where to write build artefacts. We default to the "build" directory in the
 # tox.ini file's directory. Override with the TOXINI_ARTEFACT_DIR environment
 # variable.
-build_root={env:TOXINI_ARTEFACT_DIR:{toxinidir}/build}
+build_root={env:TOXINI_ARTEFACT_DIR:{toxinidir}/build}/{envname}
 
 [testenv]
 # Which environment variables should be passed into the environment.