my-env: Use sed to substitute $initialPath.

initialPath is now added to the top of the modified setup.sh. It's not
very nice altogether to patch setup.sh in this way, as it is prone to
break on setup.sh changes, but it should (hopefully finally, but
unfortunately only temporarily) fix the issue ewemoa (in #nixos) had
with myEnvFun.

In the long term, if people want to have roughly similar functionality,
nix-shell provides a better way for that, so maybe we might either
remove myEnvFun or replace it with a smaller implementation using
nix-shell.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>