TST: fix bench workflow inputs on push trigger

A push-triggered run has an empty inputs context, so ${{ inputs.environment }}
expanded to empty and pixi consumed the task name as the --environment value.
Move defaults into a job-level env block with fallbacks so both push and
workflow_dispatch work.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jbrockmendel

.github/workflows/bench-read-csv-windows.yml

@@ -38,6 +38,16 @@ jobs:
   bench:
     runs-on: windows-2025
     timeout-minutes: 90
+    # Defaults live here (not only in the workflow_dispatch `inputs`) because a
+    # `push`-triggered run has an empty `inputs` context -- so every `inputs.x`
+    # would expand to "" without these fallbacks.  workflow_dispatch overrides
+    # them.  Job-level env propagates to every step below.
+    env:
+      BENCH_ENV: ${{ inputs.environment || 'py313' }}
+      BENCH_N_ROWS: ${{ inputs.n_rows || '2000000' }}
+      BENCH_THREADS: ${{ inputs.threads || '1 2 4' }}
+      BENCH_N_RUNS: ${{ inputs.n_runs || '7' }}
+      BENCH_N_WARMUP: ${{ inputs.n_warmup || '2' }}
     steps:
       - name: Checkout
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
@@ -47,7 +57,7 @@ jobs:
       - name: Create virtual environment with Pixi
         uses: ./.github/actions/setup-pixi
         with:
-          environment: ${{ inputs.environment }}
+          environment: ${{ env.BENCH_ENV }}
 
       - name: Remove link.EXE for Windows
         # https://github.com/Quansight-Labs/uarray/pull/310#issuecomment-3872989816
@@ -57,17 +67,13 @@ jobs:
       - name: Build pandas
         run: |
           pixi run \
-            --environment ${{ inputs.environment }} \
+            --environment ${{ env.BENCH_ENV }} \
             build-pandas \
             --editable \
             '-Csetup-args="--vsenv"'
         shell: bash -euox pipefail {0}
 
       - name: Run benchmark
-        env:
-          BENCH_N_ROWS: ${{ inputs.n_rows }}
-          BENCH_THREADS: ${{ inputs.threads }}
-          BENCH_N_RUNS: ${{ inputs.n_runs }}
-          BENCH_N_WARMUP: ${{ inputs.n_warmup }}
-        run: pixi run --environment ${{ inputs.environment }} python scripts/bench_read_csv_parallel.py
+        # BENCH_* are inherited from the job-level env block above.
+        run: pixi run --environment ${{ env.BENCH_ENV }} python scripts/bench_read_csv_parallel.py
         shell: bash -euox pipefail {0}