cmd/go/testdata/script: update test_flags for new test output
With CL 635856, an error for a package missing go files is now caught at
load time rather than at build time, so it's reported differently.
Update the test to check for the correct output unbreak the builders.
For #70820
Change-Id: I91fcac17961635b9a92782caa58c9a6f057a7e02
Reviewed-on: https://go-review.googlesource.com/c/go/+/637115 Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Auto-Submit: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>