lib: constants: change CHROMITE_BIN_DIR to Path

BUG=None
TEST=CQ passes

Change-Id: Id3885f232af7a700f33183164b43d1da4295145f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4374329
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Ram Chandrasekar <rchandrasekar@google.com>
diff --git a/scripts/cros_generate_sysroot.py b/scripts/cros_generate_sysroot.py
index b39e88a..f5a2b3c 100644
--- a/scripts/cros_generate_sysroot.py
+++ b/scripts/cros_generate_sysroot.py
@@ -64,9 +64,7 @@
 class GenerateSysroot(object):
     """Wrapper for generation functionality."""
 
-    PARALLEL_EMERGE = os.path.join(
-        constants.CHROMITE_BIN_DIR, "parallel_emerge"
-    )
+    PARALLEL_EMERGE = constants.CHROMITE_BIN_DIR / "parallel_emerge"
 
     def __init__(self, sysroot, options):
         """Initialize