internal/iapclient: check OnGCE before reading project ID
internal/iapclient: check OnGCE before reading project ID

I assumed that it would do this internally and return an error if we
weren't on GCE. Apparently I was wrong.

For golang/go#54344.

Change-Id: I186c7c5416f34682e41534810aba2f5840e69a1b
Reviewed-on: https://go-review.googlesource.com/c/build/+/422636
Reviewed-by: Jenny Rakoczy <6a3f4389a53c889b623e67f385f28ab8e84e5029@golang.org>
Auto-Submit: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>

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