nginx/etag-patch: Use Nix store dir from build env

So far, the Nix store directory was hardcoded and if someone uses a
different Nix store directory the patch won't work. Of course, this is
pretty uncommon, but by not only substituting the store directory but
also the length of it we also save a few calls to ngx_strlen(), which
should save us a few cycles.

Signed-off-by: aszlig <aszlig@nix.build>