cmd/buildlet: use sudo to enable developer mode on Macs
cmd/buildlet: use sudo to enable developer mode on Macs

Apparently Josh's intuition in CL 170339 was correct. We do need sudo.
Sometimes. It's weird. It works without sudo with gomote, but not
during start-up. The environment is slightly different.

In any case, sudo works. Verified on macOS 10.10, 10.11, 10.12, and 10.14.

Updates golang/go#31123

Change-Id: Idc4ace816a5d8e19bbb124663ca186fdf74b188c
Reviewed-on: https://go-review.googlesource.com/c/build/+/170438
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

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