Set RUST_BACKTRACE=0 in feature tests

If RUST_BACKTRACE=1 was set outside the test runner,
e.g. `RUST_BACKTRACE=1 cargo test`, this propagated to the test binary
and changed error outputs.

Interestingly this only affected `nested_json_type_error`.

Resolves #406.

tests/cli.rs

@@ -92,6 +92,7 @@ fn get_base_command() -> Command {
     cmd.env("XH_CONFIG_DIR", "");
     #[cfg(target_os = "windows")]
     cmd.env("XH_TEST_MODE_WIN_HOME_DIR", "");
+    cmd.env("RUST_BACKTRACE", "0");
     cmd
 }