Fix gum installer

[cyommer]

As noted in issue #230, the app_gum.sh script contains a fragile link to the gum installer:

# Gum is used for the Omakub commands for tailoring Omakub after the initial install
cd /tmp
GUM_VERSION="0.14.1" # Use known good version
wget -qO gum.deb "https://github.com/charmbracelet/gum/releases/latest/download/gum_${GUM_VERSION}_amd64.deb"
sudo apt-get install -y ./gum.deb
rm gum.deb
cd -

This update changes that link to always point to the version specified in the variable declaration. The updated link looks like this:

wget -qO gum.deb "https://github.com/charmbracelet/gum/releases/download/v${GUM_VERSION}/gum_${GUM_VERSION}_amd64.deb"

This has been tested on a fresh install of Ubuntu 24.04.

[dhh]

Closes #231