Rename the gprbuild index entry

[simonjwright]

The index entry for gprbuild is presently e.g. gprbuild=23.0.1-mac-aarch64, whereas the one for the compiler is gnat_macos_aarch64=13.1.1.

Should the gprbuild one instead be gprbuild_macos_aarch64=23.0.1 to match? (and, by the way, it reports itself as 23.0.0 still!)

[simonjwright]

There seems to be special logic in gnat-external.toml, or at any rate the use of it, which means that this attempt is doomed to failure. Perhaps it’s because of gnat_native-*.toml?