feat: Add --rust-version flag

[epage]

Fixes #196

[taiki-e]

The CI failure is due to --version-range being used to test compatibility with older versions of Cargo. It can be avoided by preventing passing --version-range when --rust-version is already passed.

cargo-hack/tests/auxiliary/mod.rs

Lines 63 to 67 in d74e3f7

	if let Some(toolchain) = test_version() {
	if !args.iter().any(|a| a.as_ref().to_str().unwrap().starts_with("--version-range")) {
	cmd.arg(format!("--version-range=1.{toolchain}..=1.{toolchain}"));
	}
	}

[epage]

Looks like its passing now

[taiki-e]

LGTM

[taiki-e]

Thanks!