Disable RUST_BACKTRACE as it adds 10-20 seconds to every compile

See rust-lang/rust#37477

Author: shepmaster

ui/src/sandbox.rs

@@ -217,8 +217,7 @@ impl Sandbox {
             .args(&["--net", "none"])
             .args(&["--memory", "256m"])
             .args(&["--memory-swap", "320m"])
-            .args(&["--env", "PLAYGROUND_TIMEOUT=10"])
-            .args(&["--env", "RUST_BACKTRACE=1"]);
+            .args(&["--env", "PLAYGROUND_TIMEOUT=10"]);
 
         if cfg!(feature = "fork-bomb-prevention") {
             cmd.args(&["--pids-limit", "512"]);