Specify targeting channels and add CI tests

.github/workflows/ci.yaml

@@ -99,11 +99,15 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest]
-        channel: [stable, beta, nightly]
+        rust-channel: [stable, beta, nightly]
         profile: [minimal, default]
+        nixpkgs-channel: [nixpkgs-unstable, nixos-21.11]
+        exclude:
+          - os: macos-latest
+            nixpkgs-channel: nixos-21.11
     runs-on: ${{ matrix.os }}
     env:
-      NIX_SHELL_CMD: nix-shell --pure --argstr channel ${{ matrix.channel }} --argstr profile ${{ matrix.profile }}
+      NIX_SHELL_CMD: nix-shell --pure --argstr channel ${{ matrix.rust-channel }} --argstr profile ${{ matrix.profile }}
 
     steps:
     - name: Checkout
@@ -112,7 +116,7 @@ jobs:
     - name: Install Nix
       uses: cachix/install-nix-action@v16
       with:
-        nix_path: nixpkgs=channel:nixpkgs-unstable
+        nix_path: nixpkgs=channel:${{ matrix.nixpkgs-channel }}
 
     # Workaround for https://github.com/oxalica/rust-overlay/issues/54
     - name: Don't let ~/.cargo/bin mess things up

README.md

@@ -13,6 +13,11 @@ no need to have network access. It also works well with [Nix Flakes](https://nix
 - Current oldest supported version is stable 1.29.0 and beta/nightly 2018-09-13
   (which are randomly picked and may change over time).
 
+rust-overlay is targeting these channels and they are tested on CI.
+Other channels may be supported but are not guarenteed.
+- nixpkgs-unstable
+- nixos-21.11 (for Linux)
+
 For migration from [nixpkgs-mozilla], see [this section](#migration-from-nixpkgs-mozilla).
 
 ## Installation