I had observed that 'bash --rcfile' would do nothing in a
non-interactive context and cause nothing to be executed if a script
using nix-shell shebangs were run in a non-interactive context.
The 'args' variable here is shadowing one in the outer scope and its
contents end up unused. This causes any '#! nix-shell' lines to
effectively be ignored. The intention here was to clear the args vector,
as far as I can tell (and it seems to work).
We were passing "p=$PATH" rather than "p=$PATH;", resulting in some
invalid shell code.
Also, construct a separate environment for the child rather than
overwriting the parent's.
BASH_ENV causes all non-interactive shells called via eg. /etc/bashrc to
remove the rc-file before the main shell gets to run it. Completion
scripts will often do this. Fixes#976.
Adapted from and fixes#1034.
