Set RUST_BACKTRACE=0 for rustdoc-ui/failed-doctest-output.rs

This UI test is sensitive to backtrace output, so it should make sure
that backtraces are not enabled by the environment.

src/test/rustdoc-ui/failed-doctest-output.rs

@@ -15,6 +15,7 @@
 // compile-flags:--test
 // normalize-stdout-test: "src/test/rustdoc-ui" -> "$$DIR"
 // failure-status: 101
+// rustc-env:RUST_BACKTRACE=0
 
 // doctest fails at runtime
 /// ```