get_source.sh: tune installed packages

- We need a host compiler to build tools during ACS image creation; install
  the build-essential package.
- The python package is not available anymore on Ubuntu Focal; install
  python-is-python3 on this release instead.
- We need wget to download the cross toolchains; install it.
- We need gettext to build GRUB; install it.
- We need dosfstools to generate the image; install it.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>