ci(s001): the cold-start timer used a GNU date extension and died on macOS

`date +%s%3N` is GNU-only. BSD date on the macOS runner emitted "17887458973N" and the
arithmetic failed with "value too great for base". python3 is on all three runner images and
gives the same number everywhere.

Linux passed the same step in the same run — COLD_START_MS=590, HTTP 200 on port 33395 — so
the timer works and only its portability was wrong. Third defect this workflow's runs have
surfaced that no local run could have: the first two were the sidecar's Tauri name and the
per-environment deps.get on Windows.

mix gate exit=0.

Signed-off-by: Ayla Croft <aylacroft@proton.me>

Author: HackTuah

.github/workflows/package.yml

@@ -185,7 +185,11 @@ jobs:
         shell: bash
         run: |
           set -euo pipefail
-          start=$(date +%s%3N)
+          # `date +%s%3N` is a GNU extension: BSD date on the macOS runner emitted
+          # "17887458973N" and the arithmetic died with "value too great for base". python3 is
+          # present on all three runner images and gives the same number everywhere.
+          now_ms() { python3 -c 'import time;print(int(time.time()*1000))'; }
+          start=$(now_ms)
           PHX_SERVER=true "./burrito_out/${{ matrix.artifact }}" --no-halt > serve.log 2>&1 &
           PID=$!
           for _ in $(seq 1 240); do
@@ -195,7 +199,7 @@ jobs:
           done
           test -n "${PORT:-}"
           until curl -sS -o /dev/null "http://127.0.0.1:$PORT/" 2>/dev/null; do sleep 0.05; done
-          elapsed=$(( $(date +%s%3N) - start ))
+          elapsed=$(( $(now_ms) - start ))
           code=$(curl -sS -o /dev/null -w '%{http_code}' "http://127.0.0.1:$PORT/")
           echo "HTTP $code on port $PORT"
           echo "COLD_START_MS=$elapsed" | tee -a "$GITHUB_ENV"