Test go_dmg with Homebrew create-dmg instead of local clone

[karlglazebrook]

go_dmg currently hard-codes a path to a local checkout of https://github.com/create-dmg/create-dmg at ~/Dropbox/Software/SciPDL-Own-Perl/create-dmg (currently create-dmg 1.2.0).

Homebrew also offers brew install create-dmg (currently create-dmg 1.2.3) — same upstream project, slightly newer minor version. This has not been tested with the SciPDL go_dmg flow as of v2.104.

Tasks

• brew install create-dmg
• Edit go_dmg to use the brew-installed create-dmg instead of the local clone, e.g.:

  CREATE_DMG=$(dirname $(which create-dmg))

• Run ./go_dmg end-to-end and verify the resulting .dmg looks identical to a local-clone build (icon positions, background image, Applications symlink, signing, notarisation)
• If it works cleanly, switch the documented setup to Homebrew and update BUILDING.md

Why bother

• One less manual git-clone step for new builders
• Easier to keep up to date alongside other Homebrew packages
• Removes the awkward Dropbox-path dependency in go_dmg

See BUILDING.md for current setup.