Fixes initialization of SimpleLogger in icu_datagen

[csmulhern]

The correct initialization order is

SimpleLogger::new()
    .with_level(log::LevelFilter::Info)
    .env()

Not

SimpleLogger::new()
    .env()
    .with_level(log::LevelFilter::Info)

Both SimpleLogger::env and SimpleLogger::with_level both override SimpleLogger::default_level. SimpleLogger::env should be called after SimpleLogger::with_level, so that RUST_LOG can override the default level set by SimpleLogger::with_level, if present. See: https://github.com/borntyping/rust-simple_logger/blob/fb96a82c274f92eff417904e6e819f25f3b51ccc/src/lib.rs#L153-L158

Without this change, setting RUST_LOG does not affect the log level used by icu-datagen.

[CLAassistant]

All committers have signed the CLA.

[csmulhern]

Rebased and signed CLA. Should be good to merge.