Tests: set HOME via the command env

Environmental variables set in `setenv` also affect pip invocations.
`HOME` determines where pip is placing its cache.
Setting HOME to anything virtual env-specific means that
every virtual environment is created with its own cache.
This wastes time, data and disk space.

tox.ini

@@ -50,11 +50,11 @@ envlist = py38-anki{45,46,47,48,49},py39-anki{50qt5,50qt6}
 
 [testenv]
 commands =
-    xvfb-run python -m pytest {posargs}
+    env HOME={envtmpdir}/home xvfb-run python -m pytest {posargs}
 setenv =
-    HOME={envdir}/home
     anki50qt6: DISABLE_QT5_COMPAT=1
 allowlist_externals =
+    env
     xvfb-run
 deps =
     pytest==7.1.1