cylc job script to use "bash -l" instead of sourcing profiles

[matthewrmshin]

It is only legacy reason that we have our job scripts sourcing profiles instead of calling the login shell (#!/usr/bin/bash -l or #!/usr/bin/ksh -l).

From discussion with @hjoliver via email:

Our current use of /etc/profile and .profile is technically incorrect ... Fixing it could result in a change in behavior for users with a .bash_profile who configured stuff for cylc in .profile just because they know our job scripts source it. But we could warn about that in next release.