Override default templates for install instructions

[tcompa]

This will apply e.g. to

• https://github.com/fractal-analytics-platform/fractal-ilastik-tasks
• https://github.com/fractal-analytics-platform/fractal-plantseg-tasks
• https://github.com/fractal-analytics-platform/fractal-olympus-converters

(based on #9)

[tcompa]

In the example of the plantseg task, it would come down to something like this (to be updated): https://github.com/fractal-analytics-platform/fractal-plantseg-tasks/blob/main/create_env_script.sh
Or of course a future version just describing how pixi on the Fractal server can be used with that package