yosys: avoid resynthesis on ORFS upgrade when yosys doesn't change

[oharboe]

This still works:

bazel run //sram:sdq_17x64_synth $(pwd)/tmp open_synth

Consider these two ORFS versions, there should be no need for resynthesis:

$ git diff v3.0-1696-g6927a54e v3.0-1694-g0a5008b6
diff --git a/flow/designs/sky130hd/microwatt/config.mk b/flow/designs/sky130hd/microwatt/config.mk
index 07bdd1cf1..243db9eb3 100644
--- a/flow/designs/sky130hd/microwatt/config.mk
+++ b/flow/designs/sky130hd/microwatt/config.mk
@@ -31,7 +31,7 @@ export MACRO_PLACE_CHANNEL = 200 200
 export CTS_BUF_DISTANCE = 600
 export SKIP_GATE_CLONING = 1
 
-export SETUP_SLACK_MARGIN = 0.2
+export export SETUP_SLACK_MARGIN = 0.2
 
 # This is high, some SRAMs should probably be converted
 # to real SRAMs and not instantiated as flops
$
``