Attempt nix-build instead of nixos-rebuild switch

I've encountered a few problems with attempting to support nixos-rebuild:
- the activation step requires `sudo` privileges
- the `buildkite-agent` runs on the same machine, socrates, that is rebuilding
  itself. This means that when the activation step runs, it will attempt to
  restart `buildkite-agent` when the agent is still working

I'm slowly removing places in my nix code that rely on '<bracket>' notation, so
that I no longer depend on NIX_PATH being set. I still have more work to do.

{covid-uk,sandbox}/default.nix are breaking when I attempt to run my
build-socrates.sh script locally, so I'm temporarily disabling them until I can
get CI working as I expect.

website/sandbox/covid-uk/default.nix

@@ -1,16 +0,0 @@
-{ pkgs, ... }:
-
-pkgs.stdenv.mkDerivation {
-  name = "covid-uk";
-  buildInputs = [];
-  src = builtins.path { path = ./.; name = "covid-uk"; };
-  # TODO(wpcarro): Need to run `yarn install` somehow.
-  # TODO(wpcarro): Need to run `npx tailwindcss build styles.css -o output.css`.
-  buildPhase = ''
-    mkdir -p $out
-    mkdir -p $out/node_modules/chart.js/dist
-    cp $src/node_modules/chart.js/dist/Chart.bundle.min.js $out/node_modules/chart.js/dist
-    cp $src/index.html $src/output.css $out
-  '';
-  dontInstall = true;
-}