env: use yosys package rather than symbiflow-yosys

Signed-off-by: Alessandro Comodi <acomodi@antmicro.com>

.github/workflows/ci.yml

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c75e1bf..86c12c1 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -29,7 +29,7 @@
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: [3.7]
     steps:
     - uses: actions/checkout@v2
       with:

environment.yml

diff --git a/environment.yml b/environment.yml
index 179c311..e0375b6 100644
--- a/environment.yml
+++ b/environment.yml
@@ -11,7 +11,7 @@
   - defaults
   - litex-hub
 dependencies:
-  - litex-hub::symbiflow-yosys
+  - litex-hub::yosys=0.9_5007_g2116c585
   - python
   - pip
   - pip: