add GitHub Actions to build on various targets

[bcdarwin]

(e.g. current OS X).

Willing to look into this if there is interest.

[gdevenyi]

We have been maintaining builders here, https://github.com/BIC-MNI/build_packages which will need updating for the latest versions.

Last time I tried, we couldn't fit the existing builds into Circle/Travis, but I think Github gives us more space and time.

Happy to help if you want to try to port these to github, I'm all for this.

[vfonov]

you might try to use the script I made for building conda packages - it doesn't try to build all the standard libraries from the scratch https://github.com/vfonov/conda-recipes-forge/tree/main/minc-toolkit-v2