test_runner: capture process state in entry point instead of runner

[krsnaSuraj]

This is a fix for #53867.

The run() function in the test runner directly accesses process.execArgv to find V8-only flags like --expose-gc that need propagation to child processes. This is problematic because run() is part of the public API exposed to end users via node:test and should not depend on ambient process state.

This PR moves those reads to the CLI entry point (test_runner.js) and passes them as explicit options to run():

• lib/internal/main/test_runner.js: capture process.env and process.cwd() and pass them to run() as options
• lib/internal/test_runner/runner.js (getRunArgs): use the execArgv parameter when non-empty, falling back to process.execArgv when execArgv is empty for backward compatibility
• lib/internal/test_runner/runner.js (runTestFile): use opts.env ?? {} instead of opts.env || process.env so the env precedence is driven by the options object being set
• lib/internal/test_runner/runner.js (run): default env to process.env and check 'env' in options for validation

This ensures that:

1. The CLI entry point captures the relevant process state once
2. run() can be called with explicit options by end users
3. Existing behavior is preserved -- when run() is called without execArgv, it still falls back to process.execArgv for V8 flag detection

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Review requested:

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