fix(mac): fix corrupt installer

Fixes #8533 after build agent maintenance put GNU coreutils cp on path
ahead of macOS /bin/cp. This subtly changed the behaviour of the `-R`
flag such that it copied the folder into a subfolder rather than copying
the contents of the folder into the destination folder:

Original layout:

```
Install Keyman.app/Contents/MacOS/
  applet
  textinputsource
  Keyman.app/
```

Corrupted layout:

```
Install Keyman.app/Install Keyman.app/Contents/MacOS/
  applet
  textinputsource
  Keyman.app/
```

Query: should we be rolling back the path changes so that /bin is first
on the path? I am concerned about other subtle changes in behaviour in
the macOS and iOS builds. Otherwise it feels like we may need to audit
the scripts in /mac, /ios (and possibly /common, /core, /web,
/resources) for potential changes.

Annoyed that it took me so long. Also annoying that macOS fails so
subtly when a parent folder has the .app extension in its name.