hooks/archiso: do not enable copytoram on optical disc drives

Do not set copytoram="y" if the device name contains /dev/sr (since we have no better way to match it).

Real optical disc drives and remotely emulated ones (apparently such things exist) can be slow and take too long to complete the copy process.

This change is not an issue for locally emulated optical disc drives, since they didn't gain much speed from copytoram anyway.