tools: lint version numbers in doc only in the default branch

Changelogs are sometimes not backported to release branches; this check
is useful on the `master` branch, not so much in the other branches.

Refs: nodejs#37767

.github/workflows/linters.yml

@@ -51,6 +51,7 @@ jobs:
       - name: Environment Information
         run: npx envinfo
       - name: Get release version numbers
+        if: ${{ github.event.pull_request.base.ref == github.event.pull_request.base.default_branch }}
         id: get-released-versions
         run: ./tools/node-lint-md-cli-rollup/src/list-released-versions-from-changelogs.mjs
       - name: Lint docs