Print warning message by println instead of warn to avoid missing

[Xuanwo]

sccache/src/config.rs

Lines 538 to 545 in ad97f69

	if env::var("SCCACHE_GCS_CREDENTIALS_URL").is_ok() {
	warn!("gcs deprecated_url has been deprecated");
	}
	if env::var("SCCACHE_GCS_OAUTH_URL").is_ok() {
	warn!("SCCACHE_GCS_OAUTH_URL has been deprecated");
	warn!("if you intend to use vm metadata for auth, please set correct service account intead");
	}

We print warning message by warn here, which could be missed if RUST_LOG is not set or been set to ERROR. It's better to use println instead.

[Xuanwo]

@sylvestre Can you help mark this issue as help wanted and good first issue? Thanks.