Because:
* `go-bootstrap` is a native build input of go, so it needs to have
an offset of -1. Otherwise, e.g. when building a go cross-compiler,
it will try to make go-bootstrap a cross-compiler too.
* have to specify `buildPackages` for the `stdenv` override, otherwise
`buildPackages.stdenv` will be the same as `pkgs.gcc8Stdenv`.
Changes are minor - I ended up just patching the ssl certs at the root
file, rather than trying to keep up with the various darwin changes.
The externalnetwork test helper location changed, to so I had to update
that patch as well.
- Add xcbuild as propagatedBuildInput on darwin 7e25bdba5e
