specify nimble commit to be able to test on Nim 1.6 (#88)

Author: narimiran

.github/workflows/ci.yml

@@ -9,3 +9,5 @@ on:
 jobs:
   build:
     uses: status-im/nimbus-common-workflow/.github/workflows/common.yml@main
+    with:
+      nimble-version: b920dad9ed76c6619be3ec0cfbf0dde6f9e39092