fix: Fix changelog prepending in git-cliff-release action (#147)

Author: janbuchar

git-cliff-release/action.yaml

@@ -126,7 +126,7 @@ runs:
 
         if [[ -n "${{ inputs.existing_changelog_path }}" ]]; then
           out_file=$GIT_CLIFF_WORKDIR/existing_changelog.md
-          cp "${{ github.workspace }}/${{ inputs.existing_changelog_path }}" $out_file
+          cp "$GIT_CLIFF_REPOSITORY/${{ inputs.existing_changelog_path }}" $out_file
           sed '/<!-- git-cliff-unreleased-start -->/,/<!-- git-cliff-unreleased-end -->/d' -i $out_file
 
           dump_context_args+=(--unreleased)