Ubuntu (Nix) container removed (#1827)

**EDIT**: Based on the iteration here, I realized that I could in fact
install Nix in single-user mode as part of the `setup` phase of
`bazelci.py`, meaning that a container preloaded with Nix isn't
necessary. Oops 😅

Ubuntu (Nix) container explicitly creates a `buildkite` user to match
the user mapped in when run by BuildKite and Nix is updated to run in
multi-user (daemon) mode via an `ENTRYPOINT` script.

Verified with `docker run -it --rm -u 997:997
gcr.io/bazel-public/ubuntu2204-nix python3` followed by `import certifi`
to validate Python still works, and then `docker run --rm -u 997:997
gcr.io/bazel-public/ubuntu2204-nix /bin/sh -e -c $'cd ~/\n git clone
https://github.com/ConsumingChaos/rules_rust.git\n cd rules_rust\n git
checkout wasm32_cc_info\n cd examples/nix_cross_compiling\n
CARGO_BAZEL_REPIN=true bazel build //:nix_cross_compiling'` to verify
the actual Bazel example target builds.

The `CARGO_BAZEL_REPIN=true` shouldn't be part of the normal run (I
don't think...), but that's something to be fixed in `rules_rust` and
not an issue with the container thankfully 😅
4 files changed