@jameinel while I agree with you that we shouldn’t expect /usr/bin/env to exist, if I run juju add-machine --series centos7 and ssh into the box when it comes up /usr/bin/env exists.
$ type -a /usr/bin/env
/usr/bin/env is /usr/bin/env
[ubuntu@n-c132 ~]$ /usr/bin/env python3
/usr/bin/env: python3: No such file or directory
[ubuntu@n-c132 ~]$ type -a python3
-bash: type: python3: not found