Without this change, building hplipWithPlugin on aarch64 fails with:
cp: cannot stat 'lj-aarch64.so': No such file or directory
builder for '/nix/store/p0x02ppv0n1xj46wvik3p3cn4jikmr1q-hplip-3.20.11.drv' failed with exit code 1
This is because the corresponding file in the archive is actually called
lj-arm64.so.
hplipPlatforms exists to convert between the different names; this
commit only changes the value to a correct one.
5153e0e491