ci: Fix size limit action base upload (#11124)

Noticed this is failing e.g.
https://github.com/getsentry/sentry-javascript/actions/runs/8294578489/job/22700116991,
it was an incorrect check: if there is no comparison branch, we don't
need a PR.

Author: mydea

dev-packages/size-limit-gh-action/index.mjs

@@ -187,7 +187,7 @@ async function run() {
     const githubToken = getInput('github_token');
     const threshold = getInput('threshold');
 
-    if (!comparisonBranch && !pr) {
+    if (comparisonBranch && !pr) {
       throw new Error('No PR found. Only pull_request workflows are supported.');
     }