OvmfPkg/build.sh: Use QEMU_COMMAND environment variable

If the user has set the QEMU_COMMAND environment variable,
then use it when running QEMU. This can be useful for running
OVMF with development builds of QEMU.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14825 6f19259b-4bc3-4df7-8a09-765794883524