Re-enable the Nix flake on aarch64-darwin

[krisajenkins]

Re-enable the Nix flake on aarch64-darwin.

PR #3422 added the flake but bailed out on Darwin pending a fix for
"could not find native static library rusty_v8". With v8 now on
145.0.0 (PR #4073) the build itself works on aarch64-darwin, so this
removes the builtins.abort guard and fills in the real sha256 for the
v145.0.0 rusty_v8 archive on aarch64-darwin.

The underlying bug — v8's build.rs writing librusty_v8.a outside the
locations cargo and crane treat as authoritative — already has a known
workaround in PR #3921, but that fix only lives in
.github/workflows/ci.yml and so does not protect the Nix build. This
ports the equivalent guard into the flake as a preBuild on
commonArgs: if the v8 build directory exists but librusty_v8.a is
missing, clean and rebuild just the v8 crate. With current
nixpkgs/crane the file does in fact survive the buildDepsOnly →
buildPackage handoff on aarch64-darwin, so the guard no-ops on the
happy path; it is defence-in-depth for the next time crane or nixpkgs
shifts.

x86_64-darwin and aarch64-linux still use placeholder hashes; users on
those platforms will continue to hit the existing fail-then-paste-hash
loop documented in librusty_v8.nix.

API and ABI breaking changes

None. Build-system only, no runtime change.

Expected complexity level and risk

Testing

• nix flake check --no-build passes on aarch64-darwin.
• nix build .#default produces working spacetime,
  spacetimedb-cli, and spacetimedb-standalone binaries; all three
  report spacetimedb tool version 2.3.0.
• nix build .#checks.aarch64-darwin.workspace-fmt passes.
• nix develop --command rustc --version succeeds (the command
  originally reported failing before this change).
• Confirmation from a reviewer with x86_64-linux that the change
  has not regressed the previously working platform.

[CLAassistant]

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