GHS: Remove bashism from custom command shell scripts
The GHS MULTI IDE generator uses an unnecessary bashism in its shell scripts generated for custom commands. It also sets the shell used to run the scripts to /bin/sh (see: https://gitlab.kitware.com/cmake/cmake/-/blob/v3.21.1/Source/cmGlobalGhsMultiGenerator.cxx#L279)
While many distributions use bash as /bin/sh, Debian/Ubuntu use dash, which doesn't support this bashism. This results in spammed messages every custom command:
...rule.sh: 2: [[: not found
...rule.sh: 4: [[: not found
...rule.sh: 6: [[: not foundEdited by Brad King