CI: Optional-Nix-dev-env-main: pin rev

`default.nix` pin does not work because `build.sh` overrides it to `default`.

Author: Anton-Latukha

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

@@ -22,7 +22,7 @@ env:
   ### and the other part of keys explained in `build.sh`, since those address external procedures aound the builds.
   ### Additional documentation is in Nixpkgs Haskell.lib: https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/haskell-modules/lib.nix
   ###
-  # rev: "nixos-unstable"         #  2020-09-29: NOTE: HNix default.nix currently pins the rev
+  rev: "b165ce0c4efbb74246714b5c66b6bcdce8cde175"
   cachixAccount: "hnix"
   CACHIX_SIGNING_KEY: ${{ secrets.CACHIX_SIGNING_KEY }}
   allowInconsistentDependencies: "false"