fix: strip trailing slash from DESIGN_DIR

$(dir ...) in GNU Make always appends a trailing slash, so
DESIGN_DIR ended up as e.g. ".../chameleon/". When config.mk
files use $(DESIGN_DIR)/gds/file.gds, this produced paths with
"//" instead of "/". Mind-bogglingly, this was the only
difference we could find that caused CI metric failures
(antenna_diodes_count) in PR The-OpenROAD-Project#4080.

Strip the trailing slash with $(patsubst %/,%,...) so that
replacing custom _DIR variables with DESIGN_DIR is a true no-op.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

Author: oharboe

flow/Makefile

@@ -88,7 +88,7 @@ DESIGN_CONFIG ?= ./designs/nangate45/gcd/config.mk
 export DESIGN_CONFIG
 include $(DESIGN_CONFIG)
 
-export DESIGN_DIR ?= $(dir $(DESIGN_CONFIG))
+export DESIGN_DIR ?= $(patsubst %/,%,$(dir $(DESIGN_CONFIG)))
 
 # default value "base" for FLOW_VARIANT and "." for WORK_HOME are duplicated
 # from variables.yaml and variables.mk because we need it