pwkit/environments/casa/scripting.py: run "casa" instead of "casapy"
pwkit/environments/casa/scripting.py: run "casa" instead of "casapy"

CASA 4.6.0 starts transitioning to the main user interface being launched with
the command "casa" rather than "casapy". I'm not sure for how long this
command has been provided, so this change may break backwards compatibility,
but I'd rather just commit it now and come up with workarounds later if
needed.

pkgw/pwkit