ci: Use GITHUB_OUTPUT envvar instead of set-output command (#20)

`save-state` and `set-output` commands used in GitHub Actions are deprecated and [GitHub recommends using environment files](https://github.blog/changelog/2023-07-24-github-actions-update-on-save-state-and-set-output-commands/).

This PR updates the usage of `set-output` to `$GITHUB_OUTPUT`

Instructions for envvar usage from GitHub docs:

https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#setting-an-output-parameter

.github/workflows/ci.yml

@@ -36,7 +36,7 @@ jobs:
       - name: Get pnpm store directory
         id: pnpm-cache
         run: |
-          echo "::set-output name=pnpm_cache_dir::$(pnpm store path)"
+          echo "pnpm_cache_dir=$(pnpm store path)" >> "$GITHUB_OUTPUT"
 
       - uses: actions/cache@v3
         name: Setup pnpm cache