CI: use nimble 0.20.1 (#137)

Author: nitely

.github/workflows/ci.yml

@@ -10,6 +10,7 @@ jobs:
   build:
     uses: status-im/nimbus-common-workflow/.github/workflows/common.yml@main
     with:
+      nimble-version: b920dad9ed76c6619be3ec0cfbf0dde6f9e39092
       test-command: |
         env NIMLANG=c nimble test
         env NIMLANG=cpp nimble test