CI: Travis: env: global: add presetup for chennel switching and its explanation

M  .travis.yml

.travis.yml

@@ -32,6 +32,18 @@ env:
     # NOTE: This is secure CACHIX_SIGNING_KEY=val
     - secure: "dm6I+M4+V+C7QMTpcSADdKPE633SvmToXZrTbZ7miNDGmMN+/SfHeN2ybi1+PW6oViMlbPN/7J/aEfiGjSJI8vLk72Y4uCWGmpSb8TXZLu6+whnxtZzzW8+z4tsM4048QJg7CF3N/25U8thRFgs3DqUub1Sf3nG9LrNWdz6ZcDQ="
     # NOTE: Nix by default uses nixpkgs-unstable channel
+    # NOTE: Setup for Nixpkgs revision
+    # `rev` vals in order of freshness -> cache & stability (branch causes differ):
+    # { master
+    # , commitHash
+    # , haskell-updates
+    # , nixpkgs-unstable
+    # , nixos-unstable
+    # , nixos-20.03
+    # }
+    # - rev=nixos-unstable
+    # # NOTE: Switching into Nixpkgs revision
+    # - NIX_PATH="nixpkgs=https://github.com/nixos/nixpkgs/archive/$rev.tar.gz"
     # NOTE: Enable all our tests in cabal
     - ALL_TESTS=yes