fix for new Github Action 22.04 image (#83)

- new tox 4 link requires handling
One may see the following error:
pep8: failed with pass_env values cannot contain whitespace, use comma to have multiple values in a single line, invalid values found 'http_proxy https_proxy INTERFACE_PATH'
- in image 22.04 python 3.6 and 3.7 for dropped

(cherry picked from commit 1e312c8)
(cherry picked from commit ffe7588)

tox.ini

@@ -12,7 +12,7 @@ setenv = VIRTUAL_ENV={envdir}
          CHARM_LAYERS_DIR={toxinidir}/layers
          CHARM_INTERFACES_DIR={toxinidir}/interfaces
          JUJU_REPOSITORY={toxinidir}/build
-passenv = http_proxy https_proxy INTERFACE_PATH
+passenv = http_proxy,https_proxy,INTERFACE_PATH
 install_command =
   pip install {opts} {packages}
 deps =