env: pin yosys version

This is due to a failure reported in #365

Signed-off-by: Alessandro Comodi <[email protected]>

Author: acomodi

environment.yml

@@ -19,6 +19,6 @@ channels:
   - defaults
   - litex-hub
 dependencies:
-  - litex-hub::yosys
+  - litex-hub::yosys=0.17_7_g990c9b8e1=20220512_085338_py37
   - litex-hub::surelog
   - litex-hub::iverilog