Deduplicate fingerprint logic and `check-cfg` lint config

[Urgau]

#13958 introduce a duplication in the code between the fingerprint logic and the check-cfg lint config, because of Cargo MSRV, the --check-cfg code generation is dependant on a minimum rustc version (1.79).

It would be good to consolidate

cargo/src/cargo/core/compiler/fingerprint/mod.rs

Lines 1424 to 1443 in 431db31

	// Include all the args from `[lints.rust.unexpected_cfgs.check-cfg]`
	//
	// HACK(#13975): duplicating the lookup logic here until `--check-cfg` is supported
	// on Cargo's MSRV and we can centralize the logic in `lints_to_rustflags`
	let mut lint_check_cfg = Vec::new();
	if let Ok(Some(lints)) = unit.pkg.manifest().resolved_toml().resolved_lints() {
	if let Some(rust_lints) = lints.get("rust") {
	if let Some(unexpected_cfgs) = rust_lints.get("unexpected_cfgs") {
	if let Some(config) = unexpected_cfgs.config() {
	if let Some(check_cfg) = config.get("check-cfg") {
	if let Ok(check_cfgs) =
	toml::Value::try_into::<Vec<String>>(check_cfg.clone())
	{
	lint_check_cfg = check_cfgs;
	}
	}
	}
	}
	}
	}

and

cargo/src/cargo/core/compiler/mod.rs

Lines 1363 to 1384 in 2415192

	// Also include the custom arguments specified in `[lints.rust.unexpected_cfgs.check_cfg]`
	if let Ok(Some(lints)) = unit.pkg.manifest().resolved_toml().resolved_lints() {
	if let Some(rust_lints) = lints.get("rust") {
	if let Some(unexpected_cfgs) = rust_lints.get("unexpected_cfgs") {
	if let Some(config) = unexpected_cfgs.config() {
	if let Some(check_cfg) = config.get("check-cfg") {
	if let Ok(check_cfgs) =
	toml::Value::try_into::<Vec<String>>(check_cfg.clone())
	{
	for check_cfg in check_cfgs {
	args.push(OsString::from("--check-cfg"));
	args.push(OsString::from(check_cfg));
	}
	// error about `check-cfg` not being a list-of-string
	} else {
	bail!("`lints.rust.unexpected_cfgs.check-cfg` must be a list of string");
	}
	}
	}
	}
	}
	}

in

cargo/src/cargo/util/toml/mod.rs

Line 2333 in 2415192

fn lints_to_rustflags(lints: &manifest::TomlLints) -> Vec<String> {

Originate from #13958 (comment)