openlane/user_project_wrapper: use string var

openlane/user_project_wrapper/config.json

diff --git a/openlane/user_project_wrapper/config.json b/openlane/user_project_wrapper/config.json
index 388070c..bc02342 100644
--- a/openlane/user_project_wrapper/config.json
+++ b/openlane/user_project_wrapper/config.json
@@ -27,7 +27,7 @@
     "MAGIC_ZEROIZE_ORIGIN": 0,
     "FP_SIZING": "absolute",
     "RUN_CVC": 0,
-    "UNIT": 2.4,
+    "UNIT": "2.4",
     "FP_IO_VEXTEND": "expr::2 * $UNIT",
     "FP_IO_HEXTEND": "expr::2 * $UNIT",
     "FP_IO_VLENGTH": "ref::$UNIT",
@@ -82,4 +82,4 @@
         "MAGIC_WRITE_FULL_LEF": 0,
         "FP_PDN_ENABLE_RAILS": 0
    }
-}
\ No newline at end of file
+}