GNAT FSF 13 available in Alire [preview]

Hi all,

We finally have GNAT FSF 13 builds available in Alire, in a separate branch for now. To try it, add an index like so:

alr index "--add=git+" --name=gnat_fsf_13 --before=community

Apologies for the delay, this release was particularly difficult to build for no good reasons (GitHub CI, time available, etc.). Also note that the compilers hosted on macOS x86_64 are missing, I will try to make them available ASAP.