workflows/release: checkout repository for release job.

although not documented, the github-changelog-generator seems to
access local git metadata.

> fatal: not a git repository (or any parent up to mount point /github)
> Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
> fatal: not a git repository (or any parent up to mount point /github)
> Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).

also to avoid data munging, we use the changelog from the generated
file rather than the step output.

.github/workflows/release.yml

@@ -40,16 +40,18 @@ jobs:
       - uses: actions/download-artifact@v2
         with:
           name: release-${{ github.sha }}
+      - uses: actions/checkout@v2
       - uses: heinrichreimer/[email protected]
         id: generate_changelog
         with:
           onlyLastTag: 'true'
           stripHeaders: 'true'
           stripGeneratorNotice: 'true'
+          output: CHANGELOG.md
           token: ${{ secrets.GITHUB_TOKEN }}
       - uses: softprops/action-gh-release@v1
         with:
-          body: ${{ steps.generate_changelog.outputs.changelog }}
+          body_path: CHANGELOG.md
           files: |
             *.tgz
             *.sha256