Fix prepare-devnet script on MacOS

[yHSJ]

Since commit 2a36ef7, the prepare-devnet script has been broken on MacOS. It seems that while the Linux implementation of chmod does not care about the order of flags and the mode(s), MacOS requires that flags come before the mode. Now, the prepare-devnet script should work on both macOS and Linux.

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• CHANGELOG updated or not needed
• Documentation updated or not needed
• Haddocks updated or not needed
• No new TODOs introduced or explained herafter

[v0d1ch]

Thanks @yHSJ ! @ffakenz could you check the changes as you are the only one on MacOS?