argonautica-rs: Support OptLevel z and s

[tscs37]

Using opt-level = "z" or opt-level = "s" returns a build error:

Error: ParseIntError { kind: InvalidDigit }

generated at argonautica-rs/build.rs#L69

Ideally the argon2 dependency would compile with this setting too but if z or s is specified, it should be sufficient to simply use opt-level 2 as a fallback.