/usr/bin in PATH

Currently `tox-current-env` adds `/usr/bin` to `PATH` _before_ the current `PATH` from environment:
```
$ cat my-run
#!/bin/sh
echo "PATH = $PATH"
$ cat tox.ini
[testenv]
allowlist_externals = my-run
commands = my-run
$ env - PATH=/tmp/test /usr/bin/tox-3.9 --current-env --no-provision --recreate -e py39
ROOT: tox-gh-actions won't override envlist because tox is not running in GitHub Actions
py39: remove tox env folder /tmp/test/.tox/py39
py39: commands[0]> my-run
PATH = /tmp/test/.tox/py39/bin:/usr/bin:/tmp/test
py39: OK (0.08=setup[0.04]+cmd[0.04] seconds)
congratulations :) (0.54 seconds)
$ env - PATH=/tmp/test /usr/bin/tox-3.9 --no-provision --recreate -e py39
ROOT: tox-gh-actions won't override envlist because tox is not running in GitHub Actions
py39: remove tox env folder /tmp/test/.tox/py39
py39: commands[0]> my-run
PATH = /tmp/test/.tox/py39/bin:/tmp/test
py39: OK (0.49=setup[0.46]+cmd[0.03] seconds)
congratulations :) (0.94 seconds)
$
```
This usually works okay, but in a case we need to override some command in `/usr/bin` by pointing `PATH` to some other location we simply can't.

Could you please either do not add `/usr/bin` to `PATH` at all, or add it _after_ the `PATH` from environment?

Thank you.