Request for improvement: SC2039 (and others) check POSIXLY_CORRECT

[arth1]

Example:

export POSIXLY_CORRECT=1
/bin/echo -e "foo\nbar"
( unset POSIXLY_CORRECT && /bin/echo -e "foo\nbar" )

This should return a warning for the first echo, but not the second.
As it is, it returns a warning for both if --shell=sh and none if --shell=bash