Use standard library path structure instead of plain string manipulation

[simuleite]

I encountered an error when activating Lombok on Windows:

Language server error: jdtls

oneshot canceled
-- stderr--
Error opening zip file or JAR manifest missing : /C:\Users\Myuser\AppData\Local\Zed\extensions\work\java/lombok/lombok-1.18.36.jar

I changed the line

let jar_path = format!("{prefix}/{jar_name}");

to

let jar_path = Path::new(prefix).join(&jar_name).to_string_lossy().into_owned();

I am not a Rust programmer, I don't know if it was correct!

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[simuleite]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[valentinegb]

Looks good to me. Thanks! I always forget that Windows is weird about things ^^'