emacs-app not installable on arm64 with +rsvg variant

[cmconnelly (Claire Connelly)]

emacs-app can't be installed with the +rsvg because rsvg is dependent on rust, and the rust package isn't available on arm64.

---> Computing dependencies for emacs-app Error: Cannot install librsvg for the arch 'arm64' because Error: its dependency rust only supports the arch 'x86_64'. Error: Follow ​https://guide.macports.org/#project.tickets to report a bug. Error: Processing of port emacs-app failed

This issue is probably dependent on ticket:60438