Fix detection of Command Line Tools in macOS installer

In some circumstances (to be determined), the Dynare installer thinks that CLT are installed, while they are not. The typical error message in this case (in install.log) is that git is not in the path.

Some instances of this:

• https://forum.dynare.org/t/i-can-t-download-dynare-4-6-3-in-macos/17296/
• https://forum.dynare.org/t/error-while-installing-dynare-4-6-3-mac-os-big-sur/17131/
• https://forum.dynare.org/t/problem-installing-4-6-0-on-macos-catalina/15247/20 (second poster)
• https://forum.dynare.org/t/installation-problems-of-4-6-0-on-macos/15345/20 (second poster)