process: move more dependency of environment variables to pre-execution

process: call prepareMainThreadExecution in node inspect

Since we should treat the node-inspect as third-party user code.

process: set up process warning handler in pre-execution

Since it depends on environment variables.

process: handle process.env.NODE_V8_COVERAGE in pre-execution

Since this depends on environment variable, and the worker threads do not need to persist the variable value because they cannot switch cwd.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines