Patchbots and new spkg versions

[Matthias Koeppe]

Patchbot operators may be interested in ticket https://trac.sagemath.org/ticket/26351 (needs review), which adds an optional field "upstream_url" to "checksums.ini". 

If `sage -i` is passed the additional option `--allow-upstream`, it will attempt to download from that URL if it cannot find the package on the sage mirrors. 

For builds invoked by `make`, one can use `make SAGE_SPKG="sage-spkg --allow-upstream"`.

I have used this ticket in conjunction with https://trac.sagemath.org/ticket/29087 ("Add GitHub Actions workflow for testing on various Linux distributions via Docker and macOS with homebrew"; needs review), but it could be helpful for patchbots as well.