How to add Windows environment variable into path parameter

[Zhirui-Zhang]

Operating System: Windows 10

Question:
I want to modify log4rs.yaml like this but don't know how to realize

appenders:
    rollingfile:
        kind: rolling_file
        path: "%LOCALAPPDATA%/testFolder/Logs/Proxy.log"
        encoder:
            pattern: "[{d(%Y-%m-%dT%H:%M:%S%.6f)} {h({l}):<5.5} {M}] {m}{n}"
        policy:
            kind: compound    
            trigger:
                kind: size
                limit: 5 mb
            roller:
                kind: fixed_window
                pattern: "%LOCALAPPDATA%/testFolder/Logs/Proxy-{}.log"
                base: 0
                count: 3
root:
    level: DEBUG
    appenders:
        - rollingfile

For path and pattern, I want to use system environment variable like %LOCALAPPDATA% or %USERNAME% or %USERPROFILE% (doestn't matter) instead of absolute path.
But seems like it doestn't recognize environment variable and create %LOCALAPPDATA% folder in current working path finally, how can I fix it, thanks in advance!