CI: replace nix in GitHub Actions with direct toolchains #286

GitHub Actions and nix do not play well together here, and the mcpx workflow pays for it twice.

Measured on a recent dezren39/nix mcpx run (same workflow shape as this repo): total job 10.6 min = ~0.5 min setup (checkout, nix installer, setup-go/deno/bun/node), 4.6 min unit+e2e tests run with the direct Go toolchain, then 5.4 min "Nix build" that recompiles the same code a second time inside a nix derivation.

Problems:

• mcpx.yml, nix-ci.yml and id-check.yml all install nix (DeterminateSystems/nix-installer-action) per job, then mostly use direct toolchains anyway.
• No nix store cache is configured anywhere (no cachix, no magic-nix-cache), so every run is cold.
• Known friction: intermittent ENOSPC failures in the nix flake check (see CI: nix flake check intermittently dies with 'No space left on device' in id-test/id-ci #257), sandbox/store quirks, and vendorHash repair steps that only exist because of the nix path.

Proposal: build and test with the direct toolchains in GHA (setup-go + go build/test, goreleaser where releases need it). Keep nix only for a step that genuinely needs a flake output, and name that exception in the workflow. The flake stays for local/dev use; CI should not need it.

Acceptance: no nix installer steps remain in PR CI (or each remaining one has a comment justifying it); per-PR CI wall time drops measurably; all existing checks still run.