Use default shell instead of bash on Unix-like OS

[yerke]

This Pull Request fixes/closes #2317.

It changes the following:

• Add find_default_unix_shell().
• Use default shell instead of bash on Unix-like OS.

Why we need it? macOS uses zsh by default now.

I followed the checklist:

• [ ] I added unittests
• [x] I ran make check without errors
• [x] I tested the overall application
• [x] I added an appropriate item to the changelog

[yerke]

@extrawurst @mtsgrd Do you mind taking a look? Thanks.

[extrawurst]

I suggest you run make check locally - it will run everything the CI runs (and currently fails)

[yerke]

@extrawurst thanks for letting me know. It passes now locally. Edit: I think it needs your approval to run the checks again in CI.

[extrawurst]

@yerke thank you for your contribution ❤️