CI: Nix-dev-env-main: strict build: explicitly state the compiler

Further the switch to 8.10.1 arrives.

Under 8.10.1 HNix currently can not be built strictly.

This build is strict, so it is bounded to 8.8 currently. Which is a Nixpkgs default.

Author: Anton-Latukha

.github/workflows/Optional-Nix-dev-env-main.yml

@@ -75,6 +75,7 @@ jobs:
         signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
     - name: Determined Nix-build
       env:
+        compiler: "default"
         buildStrictly: "true"
       run: ./build.sh