Update GitHub Actions

We update things to be a bit more modern:
- We bump the version of `actions/checkout`.
- We switch to using `cachix/install-nix-action`.
- We rename the job from `build` to `test` (because that's what we're
    actually doing: testing).
- We also rename the file to not be about `nodejs` (because it never
    really was, that was an implementation detail).

It would be nice to have this stuff cached somewhere, but caching with
GitHub Actions plus Nix plus Node.js takes more than a modicum of
effort, so we're not going to deal with it.

Author: joneshf

.github/workflows/nodejs.yml

@@ -0,0 +1,14 @@
+name: CI
+
+on:
+  - push
+
+jobs:
+  test:
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
+      - name: Install Nix
+        uses: cachix/install-nix-action@c134e4c9e34bac6cab09cf239815f9339aaaf84e
+      - name: Test
+        run: nix develop . --command make test