ci(nix): restore bounded 90m check timeout after e2e hang fix

agent-identity: unknown
agent-persona: generalist
agent-supervisor: unavailable
agent-tool: OMP
agent-tool-version: 18.0.3
agent-runtime: OMP 18.0.3
tooling-profile: dotfiles@e4789b0

Author: schickling-assistant

.github/workflows/nix.yml

@@ -7,10 +7,9 @@ on:
 jobs:
   check:
     runs-on: ubuntu-latest
-    # `nix flake check` builds the package plus every checks.*; on contended
-    # shared builders the budget is sized for degraded-runner periods (baseline
-    # ~35m healthy): fleet runner contention, not diff size, drives overruns.
-    timeout-minutes: 240
+    # A healthy full `nix flake check` baseline is ~35m; 90m provides
+    # headroom while still failing boundedly.
+    timeout-minutes: 90
     steps:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/determinate-nix-action@v3