ovmf: Use HOSTTOOLS' python3

ovmf: Use HOSTTOOLS' python3

Use the variable PYTHON_COMMAND to let know EDK2 base tools that python3
in the host shall be used.

In order build successfully, the host must have installed the
python3-distutils package or equivalent.

Based on an original patch by Ross Burton.

Cc: Ross Burton <ross.burton@intel.com>
Cc: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>