fix(nix): replace magic-nix-cache with Cachix (#17928)

* fix(nix): replace magic-nix-cache with Cachix

magic-nix-cache caused recurring CI failures (TwirpErrorResponse
ResourceExhausted) by hitting GitHub Actions Cache's 10 GB limit and
200 req/min rate limit. This was flagged as 'unfixable infra flake' in
#17836 but is actually a fixable architecture choice.

Switch to Cachix (dedicated binary cache, no GHA quota dependency):
- Replace DeterminateSystems/magic-nix-cache-action with cachix/cachix-action
- Add cachix-auth-token input to nix-setup composite action
- Pass CACHIX_AUTH_TOKEN secret through all three nix workflows
- continue-on-error: true so cache failures never block CI

Cache 'hermes-agent' is public at hermes-agent.cachix.org.
Devs can pull locally with: cachix use hermes-agent

* fix: correct cachix-action commit SHA pin

---------

Co-authored-by: Hermes Agent <hermes@nousresearch.com>

.github/actions/nix-setup/action.yml

@@ -1,8 +1,18 @@
 name: 'Setup Nix'
-description: 'Install Nix with DeterminateSystems and enable magic-nix-cache'
+description: 'Install Nix and configure Cachix binary cache'
+
+inputs:
+  cachix-auth-token:
+    description: 'Cachix auth token (enables push). Omit for read-only.'
+    required: false
+    default: ''
 
 runs:
   using: composite
   steps:
     - uses: DeterminateSystems/nix-installer-action@ef8a148080ab6020fd15196c2084a2eea5ff2d25 # v22
-    - uses: DeterminateSystems/magic-nix-cache-action@565684385bcd71bad329742eefe8d12f2e765b39 # v13
+    - uses: cachix/cachix-action@1eb2ef646ac0255473d23a5907ad7b04ce94065c # v17
+      with:
+        name: hermes-agent
+        authToken: ${{ inputs.cachix-auth-token }}
+      continue-on-error: true