Use correct executable name for Atom Dev.app in atom.sh on macOS

[daviwil]

Description of the Change

PR #17680 broke how atom.sh resolves the .app file for the version of Atom it intends to launch because we now name dev channel builds Atom Dev.app. This PR fixes the script to use the name of the .app file that atom.sh is contained within when /usr/local/bin/atom is symbolically linked to it.

This only appears as an issue when calling atom with a parameter like --version which follows a different code path that invokes the executable file at Atom Dev.app/Contents/MacOS/Atom Dev

Verification Process

• Symlinking /usr/local/bin/atom to atom.sh inside /Applications/Atom.app and calling atom --version returns Atom Stable info
• Symlinking /usr/local/bin/atom to atom.sh inside /Applications/Atom Dev.app and calling atom --version returns Atom Dev info
• Symlinking /usr/local/bin/atom to atom.sh inside /Applications/Atom Beta.app and calling atom --version returns Atom Beta info
• Symlinking /usr/local/bin/atom to atom.sh inside /Applications/Atom Nightly.app and calling atom --version returns Atom Nightly info

[daviwil]

Thanks @maxbrunsfeld!