Set RUST_BACKTRACE env variable

Author: ThibsG

src/test/ui/consts/const-eval/const-eval-query-stack.rs

@@ -1,6 +1,7 @@
 // compile-flags: -Ztreat-err-as-bug
 // build-fail
 // failure-status: 101
+// rustc-env:RUST_BACKTRACE=1
 // normalize-stderr-test "\nerror: internal compiler error.*\n\n" -> ""
 // normalize-stderr-test "note:.*unexpectedly panicked.*\n\n" -> ""
 // normalize-stderr-test "note: we would appreciate a bug report.*\n\n" -> ""

src/test/ui/consts/const-eval/const-eval-query-stack.stderr

@@ -1,5 +1,5 @@
 error: reaching this expression at runtime will panic or abort
-  --> $DIR/const-eval-query-stack.rs:17:28
+  --> $DIR/const-eval-query-stack.rs:18:28
    |
 LL |     let x: &'static i32 = &(1 / 0);
    |                           -^^^^^^^