bisect-kit: fix exception in prepare_chroot()

BUG=None
TEST=None

Change-Id: I7b09defdf1979e4931cf508d2867ba4d2844108b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/bisect-kit/+/2844609
Tested-by: Kuang-che Wu <kcwu@chromium.org>
Auto-Submit: Kuang-che Wu <kcwu@chromium.org>
Commit-Queue: Zheng-Jie Chang <zjchang@chromium.org>
Reviewed-by: Zheng-Jie Chang <zjchang@chromium.org>
diff --git a/bisect_kit/cros_util.py b/bisect_kit/cros_util.py
index 2963b1e..b3c6600 100644
--- a/bisect_kit/cros_util.py
+++ b/bisect_kit/cros_util.py
@@ -1499,7 +1499,7 @@
                         'ssh_keys', name)
     shutil.copy(
         path,
-        os.path.join(chromeos_root, 'chroot', 'home', os.environ('USER'),
+        os.path.join(chromeos_root, 'chroot', 'home', os.environ['USER'],
                      '.ssh'))