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 <[email protected]>
Auto-Submit: Michael Matloob <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: matloob

src/cmd/go/testdata/script/test_flags.txt

@@ -15,8 +15,8 @@ stdout '\Aok\s+example.com/x\s+[0-9.s]+\n\z'
 # Even though ./x looks like a package path, the real package should be
 # the implicit '.'.
 ! go test --answer=42 ./x
-stdout '^FAIL\t. \[build failed\]'
-stderr '^\.: no Go files in '$PWD'$'
+stdout '^FAIL\t. \[setup failed\]'
+stderr '^# \.\nno Go files in '$PWD'$'
 
 # However, *flags* that appear after unrecognized flags should still be
 # interpreted as flags, under the (possibly-erroneous) assumption that