CI: Test if build is reproducible

[alexvong243f]

I think tarball generation is reproducible after dd60b09 but it would be beneficial to check this in CI to make sure there are no regressions.

[alexvong243f]

AFAIK symbolic-*.tar.gz and symbolic-html.tar.gz are the only outputs the build generates, so it should be sufficient to check if tarball generation is reproducible.

I haven't look into this in details yet but https://reproducible-builds.org/ may have tutorials, tools or even infrastructure to assist us in setting up test environments.