Make EESSI_MODULE_UPDATE_PS1 configurable

[adammccartney]

We're testing if we can use EESSI's lmod init script as a means of initializing our module environment and came across this issue:

• The comments claim that it can be configured, but the variable is hard coded

[adammccartney]

ok, with the commit

• 12cd30a

The behavior is as follows

export EESSI_MODULE_UPDATE_PS1=0
source /cvmfs/software.eessi.io/versions/2025.06/init/lmod/bash
...
# PS1 remains unmodified

if the variable is unset, or if it is set to something other than 0, PS1 is modified according to the logic in the EESSI module.