cmd/builder: pass $HOME to child
cmd/builder: pass $HOME to child

It's needed by new os/user test.

Change-Id: I9dfe7c2a1c17e2cdc2d034c5cfc669e4aabd358d
Reviewed-on: https://go-review.googlesource.com/20480
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

proppy/build,golang/build,golang/build,golang/build,golang/build,proppy/build,proppy/build,proppy/build