readme: don't perpetuate old versions

(actually, `@v17` will still cause deprecation warnings, but I'll leave this for the upcoming release)

Author: ggreif

README.md

@@ -33,8 +33,8 @@ jobs:
   tests:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2.4.0
-    - uses: cachix/install-nix-action@v15
+    - uses: actions/checkout@v3
+    - uses: cachix/install-nix-action@v17
       with:
         nix_path: nixpkgs=channel:nixos-unstable
     - run: nix-build
@@ -52,8 +52,8 @@ jobs:
   tests:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2.4.0
-    - uses: cachix/install-nix-action@v15
+    - uses: actions/checkout@v3
+    - uses: cachix/install-nix-action@v17
       with:
         extra_nix_config: |
           access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}