drone: use Ubuntu instead of Fedora for AArch64 build
drone: use Ubuntu instead of Fedora for AArch64 build

The Ubuntu repositories don't seem to be having trouble with Docker
pull request limits, but the Fedora ones do sometimes.  I suspect
this is because drone.io has some sort of local mirror for Ubuntu
since it's the default.

nemequ/simde,nemequ/simde,nemequ/simde