Update Linux build instructions

[quellus]

The build instructions instructs installation of QT5 dependencies, then provides the build command qmake.

cool-retro-term can no longer be built with qt5. It, instead, must be built with qt6.

The wiki should be updated with the qt6 dependencies and also mention that some situations may default the qmake command to qt5, requiring the user to run qmake6 instead.

[quellus]

Ran out of time, so I'll have to keep digging at this later.

I have a huge chunk of the dependencies figured out for Ubuntu 26.04.

I created a fresh docker container and started tracking down dependencies one by one to solve whatever errors ./scrips/build-appimage.sh throws.

So far I have narrowed down:

apt update

apt install -y \
  git \
  rsync \
  wget \
  fuse3

apt install -y \
  build-essential \
  qmake6 \
  qt6-base-dev \
  qt6-declarative-dev \
  qml-qt6 \
  qml6-module-qtquick

One thing I'm not sure how to fix is that the qmake6 package installs qmake as qmake6, but the build script expects to run qmake. The work around I'm using at the moment is just to change the commands in the script to qmake6.