[CI] pin nixos/nix docker image, use experimental features flakes

.gitlab-ci.yml

 # Thanks to:
 # https://vadosware.io/post/zero-to-continuous-integrated-testing-a-haskell-project-with-gitlab/
 #
-#
-image: nixos/nix:latest
+
+# Fixing the nixos image saves CI time so it doesn't have to pull new
+#image every time (nixos/nix updates quite often).
+# image: nixos/nix:latest
+image: nixos/nix:2.22.0
 
 variables:
   STACK_ROOT: "${CI_PROJECT_DIR}/.stack-root"
@@ -38,9 +41,9 @@ compile:
       #- .spago/
       #- ${CI_PROJECT_DIR}/bun.lockb
   script:
-    - nix run .#fix-bun
+    - nix --extra-experimental-features "nix-command flakes" run .#fix-bun
     #- nix-shell shell.nix --run fix-bun
-    - nix run .#compile
+    - nix --extra-experimental-features "nix-command flakes" run .#compile
     #- nix-shell shell.nix --run compile
 
 test:
@@ -55,8 +58,8 @@ test:
       #- .spago/
       #- ${CI_PROJECT_DIR}/bun.lockb
   script:
-    - nix run .#fix-bun
+    - nix --extra-experimental-features "nix-command flakes" run .#fix-bun
     #- nix-shell shell.nix --run fix-bun
-    - nix run .#test-ps
+    - nix --extra-experimental-features "nix-command flakes" run .#test-ps
     #- nix-shell shell.nix --run test-ps