diff --git a/.github/workflows/run-builds.yml b/.github/workflows/run-builds.yml
index 9d720a5f8..92c89c9fa 100644
--- a/.github/workflows/run-builds.yml
+++ b/.github/workflows/run-builds.yml
@@ -34,25 +34,10 @@ jobs:
         run: |
           OCAMLRUNPARAM=b opam exec -- make build
 
-  build-nix-21-05:
+  build-nix-flake:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2.3.4
-      - uses: cachix/install-nix-action@v14.1
-        with:
-          nix_path: nixpkgs=channel:nixos-21.05
-      - run: nix-shell -p nix-info --run "nix-info -m"
-      - run: nix-build release.nix
-      - run: nix-shell --run "echo OK"
-
-  build-nix-unstable:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v2.3.4
-      - uses: cachix/install-nix-action@v14.1
-        with:
-          nix_path: nixpkgs=channel:nixos-unstable
-      - run: nix-shell -p nix-info --run "nix-info -m"
-      - run: nix-build release.nix
-      - run: nix-shell --run "echo OK"
-
+      - uses: cachix/install-nix-action@v15
+      - run: nix build
+      - run: nix develop -c echo ok