Update cargo-auditable integration to use .then_some()

[duckinator]

When working on #1568 I figured a way to keep optional settings from being included in serialized forms of GithubCiInfo and such.

The gist of it is, when something is dependent on a config setting:

• have it be an Option<...>,
• use #[serde(skip_serializing_if = "Option::is_none")]
• when using the value, use something along the lines of need_thing.then_some(thing)

When adding cargo-auditable, it touched almost every insta snapshot, but by using this approach in #1568 it only touches snapshots enabling the omnibor integration.

This will wind up touching all those snapshots again, but it seems like a good practice to adopt.

cargo-dist/cargo-dist/src/backend/ci/github.rs

Lines 276 to 277 in f915d87

	install_cargo_cyclonedx: Some(cargo_cyclonedx_install_strategy.dash()),
	install_omnibor: need_omnibor.then_some(omnibor_install_strategy.dash()),

cargo-dist/cargo-dist/src/backend/ci/github.rs

Lines 348 to 349 in f915d87

	install_cargo_auditable: install_cargo_auditable.to_owned(),
	install_omnibor: need_omnibor.then_some(install_omnibor.to_owned()),