Add a nix flake

[gefjon]

Description of Changes

Recently, my hacked-together env stopped working, probably due to our switching linkers. This pushed me to write a proper Nix packaging for SpacetimeDB, complete with flake and development environment.

Some things I haven't been able to figure out (or in some cases, just haven't bothered with):

• Unreal.
• C# WASI SDK.
• Exposing Git commit hash for spacetime --version.
• Making the Rust SDK's reauth test work. This test tries to write a file in the home directory, which is inaccessible in the Nix sandbox.

API and ABI breaking changes

It's a new thing we have to maintain if we're exposing it to users, but I need to maintain it anyways to be able to develop Spacetime, so...

Expected complexity level and risk

1

Testing

• Ran nix flake check locally.
• Ran nix build locally and then did spacetime start, got an apparently-responsive SpacetimeDB.
• Ran nix develop locally, then cargo build and cargo test in the dev shell.

[mamcx]

In my machine I get:

nix flake check

  >    Compiling reqwest v0.11.27
       > error: could not find native static library `rusty_v8`, perhaps an -L flag is missing?
       >
       > error: could not compile `v8` (lib) due to 1 previous error
       > warning: build failed, waiting for other jobs to finish...
       For full logs, run:
         nix log /nix/store/nm6mnvx3zsbpc33dfycvilaxbn4rsjvd-spacetimedb-test-1.6.0.drv
error: build of '/nix/store/nm6mnvx3zsbpc33dfycvilaxbn4rsjvd-spacetimedb-test-1.6.0.drv', '/nix/store/zs5myp6ws05nz94cjz1zkb2gzjw5a2s7-spacetimedb-clippy-1.6.0.drv' failed
SpacetimeDB on  HEAD (33ff957) via 🐳 orbstack is 📦 v1.6.0 via .NET v8.0.400 via  via 🦀 v1.90.0 via ❄️  impure (nix-shell-env)  🥋 wuur phoebe/nix-flake-and-env | ✅ took 33s
❯

[gefjon]

@mamcx or @jsdt - what happens if you manually download and decompress the appropriate archive file, do export RUSTY_V8_ARCHIVE=./path/to/download, and then attempt to build SpacetimeDB manually (without Nix) via spacetime build?

[gefjon]

I've been able to reproduce locally on a MacOS Nix installation, but not by building without Nix on Mac. Doing the thing described above worked fine and didn't reproduce the issue. Given that I'm blocked on this PR, I think I'm going to make the flake error if built on Darwin with a note and move on.

[Centril]

Rubber stamp!

[cloutiertyler]

Low risk PR, let's get this in to fix issues with building on nix