slugbuilder: Don't checkout origin/HEAD when cloning buildpacks
slugbuilder: Don't checkout origin/HEAD when cloning buildpacks

The Go buildpack is currently failing to clone due to a missing
.gitmodules file (see
https://github.com/heroku/heroku-buildpack-go/pull/134).

Given we checkout an explicit version, there is no need to checkout
origin/HEAD first.

Signed-off-by: Lewis Marshall <748e1641a368164906d4a0c0e3965345453dcc93@lmars.net>

lmars/flynn,philiplb/flynn,flynn/flynn,flynn/flynn,flynn/flynn,philiplb/flynn,flynn/flynn,lmars/flynn,lmars/flynn,flynn/flynn,lmars/flynn,philiplb/flynn,lmars/flynn,philiplb/flynn,philiplb/flynn