Fixed window roller expand env var

[Caleb-Hill]

This PR implements expanding env vars in file patterns in FixedWindowRoller as requested here https://github.com/estk/log4rs/issues/280 .

[estk]

@Caleb-Hill Horay! Thanks for opening a PR. I really like the change from taking a PathBuf to String, and after looking it over I think we can take this to 11. I suggest the following type for expand_env_vars.

    pub fn expand_env_vars<'str, Str>(s: Str) -> Cow<'str, str>
    where
        Str: Into<Cow<'str, str>>,
    {
        let orig_path: Cow<str> = s.into();
        let mut out_path = orig_path.clone();

The nice thing about this is that the ref gets passed all the way through so that in many cases there may be zero allocations. I know its a bit of a funny optimization, but...

There will be a few other resulting changes such as:

        let src_str = expand_env_vars(pattern.replace("{}", &i.to_string()));
        let src = Path::new(src_str.as_ref());

        let dst_str = expand_env_vars(pattern.replace("{}", &(i + 1).to_string()));
        let dst = Path::new(dst_str.as_ref());

I really appreciate your time, and thanks again!

[estk]

Additionally, as a note, i've been thinking about switching over to camino

[Caleb-Hill]

Updated it to use Cow, sorry I just got around to it. Somehow the github email ended up in my spam folder.

[Caleb-Hill]

Additionally, as a note, i've been thinking about switching over to camino

This seems like a good idea but probably a separate PR at least to my eyes.

[Caleb-Hill]

@estk Could you run the workflow and or re review my changes please?

[Caleb-Hill]

Fixed redundant clone lint issue on previous version.