fix: use npm pkg version in post-release (#428)

## PR Checklist

- [x] Addresses an existing open issue: fixes #427
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/template-typescript-node-package/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/template-typescript-node-package/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

Uses `npm pkg get version` to set the npm version in an `npm_version`
env variable per
https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#setting-an-environment-variable.
Then uses it in the comment template.

Author: JoshuaKGoldberg

.github/workflows/post-release.yml

@@ -3,6 +3,7 @@ jobs:
     name: Comment on relevant PRs and issues
     runs-on: ubuntu-latest
     steps:
+      - run: echo "npm_version=$(npm pkg get version)" >> "$GITHUB_ENV"
       - uses: apexskier/github-release-commenter@v1
         with:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
@@ -11,8 +12,8 @@ jobs:
 
             The release is available on:
 
-            * [npm package (@latest dist-tag)](https://www.npmjs.com/package/template-typescript-node-package/v/{release_tag})
-            * [GitHub release](https://github.com/JoshuaKGoldberg/template-typescript-node-package/releases/tag/v{release_tag})
+            * [GitHub releases]({release_link})
+            * [npm package (@latest dist-tag)](https://www.npmjs.com/package/template-typescript-node-package/v/${{ env.npm_version }}})
 
             Cheers! 📦🚀