ledger-agent: add `lib.platforms.linux`

ledger-agent requires `python3Packages.bleak` to build, which isn't
available on Darwin

Changed files
+1
pkgs
development
python-modules
ledger-agent
+1
pkgs/development/python-modules/ledger-agent/default.nix
···
np
mmahut
];
+
platforms = lib.platforms.linux;
};
}