add Nix to CI

Author: fhackett

.github/workflows/ci.yml

@@ -10,7 +10,10 @@ jobs:
   format_src_check:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v6
+    - uses: cachix/install-nix-action@v31
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
     - run: ./format_src_check.sh
   tests:
     runs-on: ubuntu-latest
@@ -20,11 +23,11 @@ jobs:
         golang-version: ['1.22', '1.23']
 
     steps:
-    - uses: actions/checkout@v4
-    - uses: actions/setup-go@v5
+    - uses: actions/checkout@v6
+    - uses: actions/setup-go@v6
       with:
         go-version: ${{ matrix.golang-version }}
-    - uses: actions/setup-java@v3
+    - uses: actions/setup-java@v5
       with:
         distribution: 'temurin'
         java-version: ${{ matrix.java-version }}