CI: run testsuite with THREADS=4, add step timeouts

The testsuite was running single-threaded (default THREADS=1), causing
3+ hour runs on 4 vCPU VMs. Add THREADS=4 to match VM CPU count.

Add timeout-minutes to build and test steps:
- stage0+stage1: 30 min (normally ~12 min)
- stage2: 60 min (normally ~38 min)
- testsuite: 120 min (normally ~30-60 min with 4 threads)

Author: angerman

.github/workflows/nix-ci.yml

@@ -134,6 +134,7 @@ jobs:
     # GIT_COMMIT_ID env var instead of running 'git rev-parse HEAD'.
     - name: Build stage0 + stage1
       shell: devx {0}
+      timeout-minutes: 30
       run: |
         set -eo pipefail
         make QUIET=1 CABAL_ARGS="-j2" ${{ steps.cabal-cache.outputs.cache-hit == 'true' && 'USE_SYSTEM_CABAL=1' || '' }} stage1
@@ -178,6 +179,7 @@ jobs:
 
     - name: Build stage2
       shell: devx {0}
+      timeout-minutes: 60
       run: |
         set -eo pipefail
         make QUIET=1 DYNAMIC=${{ matrix.dynamic }} CABAL_ARGS="-j2" ${{ steps.cabal-cache.outputs.cache-hit == 'true' && 'USE_SYSTEM_CABAL=1' || '' }} stage2
@@ -394,19 +396,21 @@ jobs:
 
     - name: Run testsuite
       shell: devx {0}
+      timeout-minutes: 120
       run: |
         set -eo pipefail
         # Free disk: remove .git (not needed for tests) and cabal index
         rm -rf .git
         find . -name .git -type d -exec rm -rf {} + 2>/dev/null || true
         rm -rf ~/.cabal-devx/packages/ 2>/dev/null || true
-        # Run testsuite from checkout's testsuite/ directory with dist GHC on PATH
+        # Run testsuite from checkout's testsuite/ directory with dist GHC on PATH.
+        # Use THREADS=4 to match VM vCPU count (default is 1 = serial).
         cd testsuite
         PATH=$GITHUB_WORKSPACE/_build/dist/bin:$PATH \
         METRICS_FILE=$GITHUB_WORKSPACE/_build/test-perf.csv \
         SUMMARY_FILE=$GITHUB_WORKSPACE/_build/test-summary.txt \
         JUNIT_FILE=$GITHUB_WORKSPACE/_build/test-junit.xml \
-        make
+        make THREADS=4
 
     - name: Stop metrics collection
       if: ${{ !cancelled() }}