Fall back to edk2-202011-18 OVFM files

Fall back to previous release due to unstable OVMF files.
Commit b97dc4b92ba1cc9f351854aed1c35c636d2d3992 in edk2 causes
qemu kvm reboots to become unstable for bootloader-extras
and performance-tools.

Signed-off-by: Mark D Horn <mark.d.horn@intel.com>

Author: mdhorn