Record environmentVariables usage in apify push telemetry #1567

The PRD's leading indicator is the share of pushes that have environmentVariables in actor.json. The cli_command event records only actorId and wasCreated for a push, so the baseline can't be measured yet. This has to ship before apify push stops uploading env vars.

Scope

• Extend CliCommandEvent.push (src/lib/hooks/telemetry/trackEvent.ts) with envVarsCount, the number of keys in environmentVariables (0 when the key is absent), and secretRefsCount, the number of values that are @secret references.
• Set both in src/commands/actors/push.ts. For a monorepo push, record the totals across the pushed Actors.
• Send counts only, never names or values.

Done when

• The push tests assert the new properties.
• Mixpanel can chart the share of pushes with envVarsCount > 0.