fix(bolt): correctly check for existing env var (#705)

<!-- Please make sure there is an issue that this PR is correlated to. -->

## Changes

<!-- If there are frontend changes, please include screenshots. -->

lib/bolt/core/src/dep/cargo/cli.rs

@@ -88,7 +88,7 @@ pub async fn build<'a, T: AsRef<str>>(ctx: &ProjectContext, opts: BuildOpts<'a,
 		#!/bin/bash
 		set -euf
 
-		[ -z "$CARGO_TARGET_DIR" ] && export CARGO_TARGET_DIR=$(readlink -f ./target)
+		[ -z "${{CARGO_TARGET_DIR+x}}" ] && export CARGO_TARGET_DIR=$(readlink -f ./target)
 		# Used for Tokio Console. See https://github.com/tokio-rs/console#using-it
 		export RUSTFLAGS="--cfg tokio_unstable"
 		# Used for debugging