ci: Use latest nixci

Also build only master branch, while still building PRs.

Author: srid

.github/workflows/ci.yaml

@@ -2,7 +2,7 @@ name: "CI"
 on:
   push:
     branches:
-      - '*'
+      - master
   pull_request:
 jobs:
   checks:
@@ -12,7 +12,7 @@ jobs:
       - name: Build Nix
         id: build-nix
         run: |
-          nix shell github:nixos/nix/2.18.1 github:srid/nixci -c nixci
+          nixci
       - name: Run test
         id: test
         run: |