boards: x86: update build_grub.sh for Python 3

Since Python 2 is already EOL, Linux distributions are starting
to drop support for Python 2. For example, on Ubuntu 20.04,
"python" is no longer available. This breaks the bootstrap
and autogen.sh as they are both default to executing "python".
Fix this by setting the PYTHON environment variable before
building so both scripts will use python3 instead.

Also update git checkout to grab the official 2.04 release.

Fixes #25398

Signed-off-by: Daniel Leung <daniel.leung@intel.com>