Use RA_LOG instead of RUST_LOG for logging

RUST_LOG might be set up for debugging the user's problem, slowing
down rust-analyzer considerably. That's the same reason why rustc uses
RUSTC_LOG.

crates/rust-analyzer/src/bin/main.rs

@@ -60,7 +60,7 @@ fn main() -> Result<()> {
 
 fn setup_logging() -> Result<()> {
     std::env::set_var("RUST_BACKTRACE", "short");
-    env_logger::try_init()?;
+    env_logger::try_init_from_env("RA_LOG")?;
     ra_prof::init();
     Ok(())
 }