Update CMake to 4.1.2

Modularize into functions

Update dmgbuild to 1.6.5

Do not cleanup PKG_DIR

This is necessary for a stable/reproducible build if some package is
no longer available.

Update JHBuild to c0d3555d

Fix comment about canonical release source

Remove global SHADOW_FILE variable

Turn it into a local variable.
Use local dmg variable to populate it, fixing incorrect naming
(suffix) in the process.

Make unmount_dmg look for the exact partition

This avoids conflicts when multiple images are being used in parallel.

Add config helper

Provide an easy way to retrieve settings from JHB's environment.

Apply updates from gtk-osx