nix-shell --pure: Let it work for any derivation

`nix-shell --pure` when applied to a non stdenv derivation doesn't seem
to clear the PATH. It expects the stdenv/setup file to do so.

This adds an explicit `unset PATH` by nix-build.cc (nix-shell) itself so
that it's not reliant on stdenv/setup anymore.

This does not break impure nix-shell since the PATH is persisted as the
variable `p` prior in the bash rcfile

fixes #5092

src/nix-build/nix-build.cc

@@ -489,6 +489,9 @@ static void main_nix_build(int argc, char * * argv)
                     "_nix_shell_clean_tmpdir; ") +
                 (pure ? "" : "[ -n \"$PS1\" ] && [ -e ~/.bashrc ] && source ~/.bashrc;") +
                 "%2%"
+                // always clear PATH.
+                // when nix-shell is run impure, we rehydrate it with the `p=$PATH` above
+                "unset PATH;"
                 "dontAddDisableDepTrack=1;\n"
                 + structuredAttrsRC +
                 "\n[ -e $stdenv/setup ] && source $stdenv/setup; "