Fix errors caught by go vet in compliance package

`go test` implicitly runs `go vet` and fails the tests if vet
errors are found.  Fix all the issues found by vet.

Test: go test build/make/tools/compliance/...
Change-Id: If0684cf124ece4931af440008cd44a61c22de475

tools/compliance/readgraph_test.go

@@ -88,9 +88,9 @@ func TestReadLicenseGraph(t *testing.T) {
 			lg, err := ReadLicenseGraph(tt.fs, stderr, tt.roots)
 			if err != nil {
 				if len(tt.expectedError) == 0 {
-					t.Errorf("unexpected error: got %w, want no error", err)
+					t.Errorf("unexpected error: got %s, want no error", err)
 				} else if !strings.Contains(err.Error(), tt.expectedError) {
-					t.Errorf("unexpected error: got %w, want %q", err, tt.expectedError)
+					t.Errorf("unexpected error: got %s, want %q", err, tt.expectedError)
 				}
 				return
 			}