fix(cli.rs): conflicts_with arg doesn't exist closes

[JonasKruckenberg]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Docs
• New Binding issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes, and the changes were approved in issue #___
• No

Checklist

• When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary

Other information

If you run the command cargo run -- signer sign --help using the latest master commit it will panick with the following message:

$ cargo run -- signer sign --help
   Compiling tauri-cli v1.0.1 (/Users/jonaskruckenberg/Documents/GitHub/tauri/tooling/cli)
    Finished dev [unoptimized + debuginfo] target(s) in 6.01s
     Running `target/debug/cargo-tauri signer sign --help`
thread 'main' panicked at 'Command sign: Argument or group 'private_key_path' specified in 'conflicts_with*' for 'private-key' does not exist', /Users/jonaskruckenberg/.cargo/registry/src/github.com-1ecc6299db9ec823/clap-3.2.7/src/builder/debug_asserts.rs:237:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This PR adds a quick fix for this (cherry picked from #4537)