Using an FSO build selected by browsing with "FSO Settings" doesn't work on macOS

[jg18]

Found with Knossos 0.14.3 on macOS Catalina 10.15.4.

If I select an FSO build for "Retail FS2" by browsing to a specific FSO .app through the "FSO Settings" dialog, then after I press "SAVE", close the selection popup and then click on PLAY, I see a dialog with the message, "I can't find the selected executable! (The file "//fs2_open_.app" is missing!)"

The problem might be that this method for selecting an FSO executable doesn't account for macOS using app bundles to represent executables, but that's just a guess.