fix: honour BB_BINARY_PATH (#23570)

.

Author: alexghr

barretenberg/ts/src/bb_backends/node/platform.ts

@@ -83,7 +83,8 @@ export function detectPlatform(): Platform | null {
  *
  * Search order:
  * 1. If customPath is provided and exists, return it
- * 2. Otherwise search in <package-root>/build/<platform>/bb
+ * 2. If BB_BINARY_PATH is set and exists, return it
+ * 3. Otherwise search in <package-root>/build/<platform>/bb
  */
 export function findBbBinary(customPath?: string): string | null {
   // Check custom path first if provided
@@ -95,6 +96,14 @@ export function findBbBinary(customPath?: string): string | null {
     return null;
   }
 
+  const envPath = process.env.BB_BINARY_PATH;
+  if (envPath) {
+    if (fs.existsSync(envPath)) {
+      return path.resolve(envPath);
+    }
+    return null;
+  }
+
   // Automatic detection
   const platform = detectPlatform();
   if (!platform) {