Modified the staging_install script so that an option "-variable"
can be passed, such that .mag files in mag/ and maglef/ directories
of the libraries will have GDS files point to the PDK by variable
reference, e.g., using $PDK_PATH, so that layouts can be made
portable.

sky130/Makefile.in

diff --git a/sky130/Makefile.in b/sky130/Makefile.in
index faf5480..194b5e6 100644
--- a/sky130/Makefile.in
+++ b/sky130/Makefile.in
@@ -528,6 +528,7 @@
 	echo "Starting SKY130 PDK migration on "`date` > ${SKY130A}_migrate.log
 	${INSTALL} -source ${STAGING_PATH}/${SKY130A} \
 		-target ${LOCAL_PATH}/${SKY130A} \
+		-variable PDKPATH \
 		-link_from ${LINK_TARGETS} |& tee -a ${SKY130A}_migrate.log
 	echo "Ended SKY130 PDK migration on "`date` >> ${SKY130A}_migrate.log
 
@@ -537,6 +538,7 @@
 	echo "Starting SKY130 PDK migration on "`date` > ${SKY130A}_migrate.log
 	${INSTALL} -source ${STAGING_PATH}/${SKY130A} \
 		-target ${DIST_PATH}/${SKY130A} \
+		-variable PDKPATH \
 		-local ${LOCAL_PATH}/${SKY130A} \
 		-link_from ${DIST_LINK_TARGETS} |& tee -a ${SKY130A}_migrate.log
 	echo "Ended SKY130 PDK migration on "`date` >> ${SKY130A}_migrate.log