set a default for smithy-go repo var since it doesn't propagate to PRs from forks (#2238)

Author: lucix-aws

ci-find-smithy-go.sh

@@ -10,8 +10,7 @@
 #  - feat-foo
 
 if [ -z "$SMITHY_GO_REPOSITORY" ]; then
-    echo env SMITHY_GO_REPOSITORY is required
-    exit 1
+    SMITHY_GO_REPOSITORY=aws/smithy-go
 fi
 
 if [ -z "$RUNNER_TMPDIR" ]; then