Allow SHARED_PDKS_PATH to be set externally.

sky130/Makefile.in

diff --git a/sky130/Makefile.in b/sky130/Makefile.in
index ad9d230..104364b 100644
--- a/sky130/Makefile.in
+++ b/sky130/Makefile.in
@@ -165,7 +165,7 @@
 
 STAGING_PATH = $(shell pwd)
 
-SHARED_PDKS_PATH = $(datadir)/pdk
+SHARED_PDKS_PATH ?= $(datadir)/pdk
 
 # If LINK_TARGETS is set to "none", then files are copied
 # from the SkyWater sources to the target.  If set to "source",