Modified the magic device generator script to honor environment
variable PDK_ROOT, although the variable does not appear to be
used, so it might not make any difference.

sky130/magic/sky130.tcl

diff --git a/sky130/magic/sky130.tcl b/sky130/magic/sky130.tcl
index bce3ec2..85af9ee 100644
--- a/sky130/magic/sky130.tcl
+++ b/sky130/magic/sky130.tcl
@@ -9,7 +9,9 @@
 # Revision 1	ALPHA   9/10/2020
 #-----------------------------------------------------
 
-set TECHPATH STAGING_PATH
+if {[catch {set TECHPATH $env(PDK_ROOT)}]} {
+    set TECHPATH "STAGING_PATH"
+}
 if [catch {set PDKPATH}] {set PDKPATH ${TECHPATH}/TECHNAME}
 set PDKNAME TECHNAME
 # "sky130" is the namespace used for all devices