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 <bradfitz@golang.org>

cmd/builder/env.go

diff --git a/cmd/builder/env.go b/cmd/builder/env.go
index 596f404..2f7e348 100644
--- a/cmd/builder/env.go
+++ b/cmd/builder/env.go
@@ -297,6 +297,7 @@
 		"PATH",
 		"TMPDIR",
 		"USER",
+		"HOME",
 		"GO_TEST_TIMEOUT_SCALE", // increase test timeout for slow builders
 	}
 	if runtime.GOOS == "plan9" {