build: skip non existing IB folder removal

Typo would just check if the path was a non-empty-string instead of
checking if it's an actually existing folder.

Signed-off-by: Paul Spooren <mail@aparcar.org>

asu/build.py

@@ -396,7 +396,7 @@ def download_file(filename: str, dest: str = None):
             job.connection.hdel(
                 f"worker:{job.worker_name}:last_build", target_subtarget
             )
-            if cache / target_subtarget:
+            if (cache / target_subtarget).exists():
                 rmtree(cache / target_subtarget)
         else:
             log.debug("Keeping ImageBuilder for %s", target_subtarget)