BCR: allow users to skip the url stability check themselves

writing `@bazel-io unstable_ack` as a comment automatically applies the `skip-url-stability-check` label.

actions/release-helper/index.js

diff --git a/actions/release-helper/index.js b/actions/release-helper/index.js
index 54cf0ff..158aada 100644
--- a/actions/release-helper/index.js
+++ b/actions/release-helper/index.js
@@ -81,6 +81,13 @@
       issue_number: payload.issue.number,
       labels: [FLAGGED_LABEL],
     });
+  } else if (command === "unstable_ack" || command.startsWith("unstable_ack ")) {
+    await octokit.rest.issues.addLabels({
+      owner,
+      repo,
+      issue_number: payload.issue.number,
+      labels: ["skip-url-stability-check"],
+    });
   } else {
     await octokit.rest.reactions.createForIssueComment({
       owner,