Remove linuxbrew from path when detecting shell

Addresses issue: pypa#6197

This updates the check to only look at the path after linuxbrew if it is
present.

pipenv/shells.py

@@ -35,6 +35,11 @@ def _get_activate_script(cmd, venv):
  This is POSIX-only at the moment since the compat (pexpect-based) shell
  does not work elsewhere anyway.
  """
+
+ # Split on linuxbrew in case users have installed there shell using it.
+ # Otherwise the `nu` substring will trigger the nu shell path.
+ cmd = cmd.split(".linuxbrew")[-1]
+
  # Suffix and source command for other shells.
  # Support for fish shell.
  if "fish" in cmd: