If no GOIOS_DEV_ID is set, iostest.bash will eval the output of
detect.go. Prepend the note output by detect.go with # to make
the shell ignore it.

Went undetected for so long because the iOS builders usually run
with GOIOS_DEV_ID set.

Change-Id: I308eac94803851620ca91593f9a1aef79825187f
Reviewed-on: https://go-review.googlesource.com/c/144109
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>