stdenv: fix HOST_PATH change

a4630c65 was incorrect in assuming $SHELL would be a path to the
bash derivation. In fact $SHELL will be a path to the bash executable.

Unfortunately this did not fix the original issue. So instead, we just
have to reuse initialPath can be added like PATH is.

Sorry for the inconvenience! I hadn’t thought through the effects of
the last commit.

/cc @copumpkin @ericson2314