Update install.sh: handle when python is not installed

[deftdawg]

Working on bring exo up on my Steam Deck, install.sh didn't handle Python3 not being installed...
Refactored install.sh to handle that better, test below:

# ./install.sh # No python3
Python 3.12 is not installed, please install Python 3.12

# nix profile install nixpkgs#python310
# ./install.sh # python3.10
recommended version of Python to run exo with is Python 3.12, but Python 3.10.16 is installed. Proceeding with Python 3.10.16

# nix profile remove python310
# nix profile install nixpkgs#python312
# ./install.sh # python3.12
Python 3.12 is installed, proceeding with python3.12...