Always emit a set GOROOT
Always emit a set GOROOT

Tools like gimme are the reason that the `$GOROOT` env override exists.
We should always set it, so that when `GOTOOLDIR` commands (`cgo`,
`pprof`, etc) are invoked, the correct ones are used.

Unconditionally put GOROOT into the `.env` files, and fake it up from
older env files.

Fixes #87

travis-ci/gimme,glevand/travis-ci--gimme,travis-ci/gimme,glevand/travis-ci--gimme,meatballhat/gimme