github: make release build create staging directory

I think 672f9e85cb was correct in saying that we don't need the
`$staging/complete` directory, but we do seem to need the `$staging`
directory, so let's restore the code for creating that. While at it, I
also cleaned up a bit so we use the `$outdir` variable instead of
duplicating it.

.github/workflows/release.yml

@@ -49,16 +49,17 @@ jobs:
     - name: Build archive
       shell: bash
       run: |
-        outdir="./target/${{ matrix.target }}/release"
+        outdir="target/${{ matrix.target }}/release"
         staging="jj-${{ github.event.release.tag_name }}-${{ matrix.target }}"
+        mkdir "$staging"
         cp {README.md,LICENSE} "$staging/"
         if [ "${{ matrix.os }}" = "windows-2022" ]; then
-          cp "target/${{ matrix.target }}/release/jj.exe" "$staging/"
+          cp "$outdir/jj.exe" "$staging/"
           cd "$staging"
           7z a "../$staging.zip" .
           echo "ASSET=$staging.zip" >> $GITHUB_ENV
         else
-          cp "target/${{ matrix.target }}/release/jj" "$staging/"
+          cp "$outdir/jj" "$staging/"
           tar czf "$staging.tar.gz" -C "$staging" .
           echo "ASSET=$staging.tar.gz" >> $GITHUB_ENV
         fi