dashboard/builder: pass GOROOT_BOOTSTRAP to child processes

Part of https://golang.org/s/go15bootstrap

This is one step in a series of CLs to make the dev.cc branch build
after https://golang.org/cl/2470 (which converted cmd/dist from C to
Go)

TBR to adg because subsequent changes to Dockerfiles depend on knowing
the commit hash of this one.

Change-Id: I26122e2446c345446bbbca0df601f50ea9383adb
Reviewed-on: https://go-review.googlesource.com/2651
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed