server: git push for binary transparency in `fdroid server update`

Since `fdroid server update` is the place where all uploads to servers
happens, it makes sense to also handle the git push for the binary
transparency log here instead of `fdroid btlog`
1 job for nail-down-btlog-and-bugfix in 19 minutes and 1 second (queued for 5 seconds)
Status Job ID Name Coverage
passed #14079699