github-1.0: Use only MacPorts mirrors for "downloads"

GitHub no longer hosts the files for the old "uploads" feature which was
accessible via the "downloads" URL so fetch these files only from
MacPorts mirrors now.

_resources/port1.0/group/github-1.0.tcl

@@ -30,7 +30,8 @@ proc handle_tarball_from {option action args} {
         github.tarball_from ${args}
         switch ${args} {
             downloads {
-                github.master_sites https://github.com/downloads/${github.author}/${github.project}
+                # GitHub no longer hosts downloads on their servers.
+                github.master_sites macports_distfiles
                 default extract.rename no
             }
             releases {