Ignore deprecation warning in clap macro invocation

[faern]

Maybe about time to get rid of the annoying:

warning: use of deprecated item 'std::sync::ONCE_INIT': the `new` function is now preferred

  --> mullvad-daemon/src/cli.rs:69:17
   |
69 |         .author(crate_authors!(", "))
   |                 ^^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(deprecated)]` on by default
   = note: this warning originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

I have realized that clap will never fix the warning. Simply because that would bump the MSRV, and clap 2 has apparently stated that it should remain at 1.24 🤷‍♂️ :( (clap-rs/clap#1988)

So I rearranged the code a little bit in order to be able to stick an #[allow(deprecated)] on the problematic code, but just as little of it as possible.

We use the same macro in mullvad-cli. But for some reason that does not emit any warning during regular build, so I did not touch it. I don't know why and I did not spend any time on figuring it out either.

Git checklist:

• Describe the change in CHANGELOG.md under the [Unreleased] header.
• Check that commits follow the Mullvad coding guidelines

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This change is 

[jvff]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved