Make EESSI_MODULE_UPDATE_PS1 configurable

[adammccartney]

Fixes #187

[ocaisa]

There were changed today and I suggest you start again from https://github.com/EESSI/software-layer-scripts/blob/main/init/lmod/sh.

General approach looks ok, but if you call the script twice the EESSI_MODULE_UPDATE_PS1 will be set on the second call. You probably want a structure similar to

if [ -z "$__Init_EESSI_PS1" ]; then
  export __Init_EESSI_PS1=1;
  ...
fi

You should also add a test for this case near https://github.com/EESSI/software-layer-scripts/blob/main/.github/workflows/scripts/test_init_scripts.sh#L145-L153

[ocaisa]

For the test it could be

TEST_EESSI_EXPLICIT_NO_PS1_UPDATE=$($shell -c "unset PS1 ; PS1='$ ' ; export EESSI_MODULE_UPDATE_PS1=0 ; . init/lmod/$shell 2>/dev/null ; echo \"\$PS1\"")
...
assert_raises 'echo "$TEST_EESSI_EXPLICIT_NO_PS1_UPDATE" | grep "$pattern"' 1