* Don't look for gprbuild from system provider.
Because the manifest currently asks for Alire to attempt to download
gprbuild from the system package manager, alr <anything> spends
several fruitless seconds trying to find gprbuild.
* index/gp/gprbuild/gprbuild-external.toml: remove the 'system' external.
* macOS distributions don't provide gprbuild, others do.
* index/gp/gprbuild/gprbuild-external.toml: the "system" external isn't
available on macOS, so specify it explicitly for the other operating
systems.
Simon Wright
df12ae1d76