Change RT_CLOCK_MIN_LAYER to met3 for sky130

sky130/openlane/config.tcl

diff --git a/sky130/openlane/config.tcl b/sky130/openlane/config.tcl
index 9700132..25b7f5e 100755
--- a/sky130/openlane/config.tcl
+++ b/sky130/openlane/config.tcl
@@ -181,5 +181,8 @@
 set ::env(RT_MIN_LAYER) "met1"
 set ::env(RT_MAX_LAYER) "met5"
 
+set ::env(RT_CLOCK_MIN_LAYER) "met3"
+
 # CVC
 set ::env(CVC_SCRIPTS_DIR) "$::env(PDK_ROOT)/$::env(PDK)/libs.tech/openlane/cvc"
+