test: lower swerv PLACE_DENSITY_LB_ADDON 0.25 -> 0.20 for GRT-0116

Build #4 on secure-test-clamp-only-with-rules surfaced nangate45/swerv
GRT-0116 routing congestion not present on pre-clamp baseline. Clamped
port buffers concentrate local density; with util=65% and addon=0.25 the
target density forces packing too tight for routing.

Formula (flow/scripts/util.tcl:179):
  target_density = util + (1 - util) * addon + 0.01
Before: 0.65 + 0.35 * 0.25 + 0.01 = 0.7475
After:  0.65 + 0.35 * 0.20 + 0.01 = 0.7300

Modest reduction matching gf180/jpeg level. If still congested, drop
further toward sky130hd/jpeg (0.15) or ihp-sg13g2/ibex (0.10).

Signed-off-by: minjukim55 <mkim@precisioninno.com>

Author: minjukim55

flow/designs/nangate45/swerv/config.mk

@@ -8,7 +8,7 @@ export CORE_UTILIZATION = 65
 export CORE_ASPECT_RATIO = 1
 export CORE_MARGIN = 5
 
-export PLACE_DENSITY_LB_ADDON = 0.25
+export PLACE_DENSITY_LB_ADDON = 0.20
 export TNS_END_PERCENT        = 100
 
 export SWAP_ARITH_OPERATORS = 1