cmd/go: fix TestScript/list_ambiguous_path on Plan 9

CL 198459 added TestScript/list_ambiguous_path. This
test is failing on Plan 9, because the expected error
doesn't match the error message returned on Plan 9.

This change fixes the test by matching the correct
error message on Plan 9.

Fixes #35072.

Change-Id: If8cdb641e0e9544ae4ac24f8d0c54859a3b23a69
Reviewed-on: https://go-review.googlesource.com/c/go/+/202447
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

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

@@ -22,8 +22,9 @@ stderr 'package ./foo.go/b.go: cannot find package "."'
 # Multiple patterns for Go files with a typo. This should
 # treat the wrong pattern as if it were a non-existint file.
 ! go list ./foo.go/a.go ./foo.go/b.go
+[plan9] stderr 'stat ./foo.go/b.go: ''./foo.go/b.go'' does not exist'
 [windows] stderr './foo.go/b.go: The system cannot find the file specified'
-[!windows] stderr './foo.go/b.go: no such file or directory'
+[!plan9] [!windows] stderr './foo.go/b.go: no such file or directory'
 
 -- a.go --
 package main