feat: add repeatable --env flag to push (#734)

[MatousMarik]

Completes #734, stacked on #1346 (retarget to master as the stack merges — only the last two commits are new here).

What

• Repeatable --env KEY=VALUE flag on apify push, primarily for passing build args / env vars in CI without touching actor.json or the Console.
• Values are merged with environmentVariables from .actor/actor.json; on key conflict the --env value wins. @secret references resolve the same way as in the file (via apify secrets add). Malformed values (no KEY=) fail fast with exit code 5 before anything is uploaded.
• Framework (own commit): Flags.string({ multiple: true }) — multi-value string flags typed as string[]. The parser already registered every flag with multiple: true and collapsed the array in _parseFlags; flags tagged multi-value now keep it. Includes the 'strings' flag tag in type inference and both help renderers. choices and default are type-forbidden together with multiple: true (unimplemented in parsing); The short-form -x=value leading-= strip is applied per element for multi-value flags (unit-tested); stdin with multiple remains a known ceiling.

Resulting env var semantics

environmentVariables in actor.json	--env	Env vars on the platform after push
absent	none	Preserved (field omitted from the API call — unchanged behavior)
absent	--env A=1	Replaced with the CLI vars — Console-set vars are removed (documented with a caution in vars.md and the flag description)
{...}	none	Replaced by the file's vars (unchanged behavior)
{} (empty object)	none	Cleared (unchanged behavior — an explicit empty value)
{...}	--env ...	Replaced by the merge; CLI wins per key

Tests

• Unit tests for the KEY=VALUE parser (merge order, = in values, empty value, malformed entries).
• Unit tests for multi-value flag parsing (repeated values collect, scalars wrap, absent stays undefined, single-value flags still reject repeats).
• New [api] tests: file + CLI vars merge with CLI winning; @secret refs from both actor.json and --env land as isSecret: true and never come back in plain text; platform vars preserved when the field is absent and cleared by an explicit {} (pins the table above). Full push API suite passes (17/17), plus test:local, lint, format, build.
• Manually verified the real argv path: repeated --env parses, malformed value exits 5.

No new dependencies; no install-size impact.