temporarily pin clap version

[FabianLars]

For whatever reason, cargo resolves beta.5 to rc.0 (it didn't do that between beta releases). This breaks both the released cli and the next version mainly because they completely removed the yaml api.
So if we rewrite the cli to use the builder or derive api soon enough, then i don't expect this to be merged, but at least i can tell users with this problem to install the cli from this branch instead.

Edit: Nevermind they only deprecated the yaml feature for now. I'll take a look on what they changed that breaks our cli.

Edit2 (from discord): "tbf the fixes are pretty easy (another feature flag for macros, App::from -> App::from_yaml, arg.hidden -> arg.hide), but i don't know if it's worth it if the yaml api is supposedly so bad (clap-rs/clap#3087)"

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Does this PR introduce a breaking change? (check one)

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[epage]

For whatever reason, cargo resolves beta.5 to rc.0 (it didn't do that between beta releases).

Yes, this is a bit annoying that preleases are considered semver compatible with each other. See https://internals.rust-lang.org/t/changing-cargo-semver-compatibility-for-pre-releases/14820/2

[lucasfernog]

We've updated clap to 3.0.0-rc.0 instead: #2928