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.

ci/scripts/build-socrates.sh

@@ -1,8 +1,6 @@
 set -euo pipefail
 
-nixos-rebuild \
-  -I nixos-config=/home/wpcarro/briefcase/nixos/socrates/configuration.nix \
-  -I nixpkgs=/home/wpcarro/nixpkgs \
-  -I depot=/home/wpcarro/depot \
-  -I briefcase=/home/wpcarro/briefcase \
-  switch
+nix-build /home/wpcarro/nixpkgs/nixos \
+  -I nixos-config=/home/wpcarro/briefcase/nixos/socrates/default.nix \
+  -A system \
+  --show-trace