add $ at the end of test

Author: Yuuki77

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

@@ -1,5 +1,5 @@
 ! go build
-stderr 'main.go:4:2: local import "./foo" not supported in module mode'
+stderr 'main.go:4:2: local import "./foo" not supported in module mode$'
 
 -- foo/foo.go --
 package foo