fix: let --loglevel on the command line override npm_config_loglevel

[gengjiawen]

Checklist

• npm install && npm run lint && npm test passes
• tests are included
• commit message follows commit guidelines

Description of change

Fixes the citgm node-gyp failure reported in nodejs/citgm#1153. It fails the same 3 tests on every OS and Node.js 22/24/26 (run):

• addon › build simple addon
• addon › addon works with renamed host executable
• options › options in environment

Root cause

citgm always sets npm_config_loglevel (default error) in the environment of the module's npm test. Since #3196 (v11.4.0), parseArgv() copies npm_config_* into opts after parsing the command line, so the inherited npm_config_loglevel replaces an explicit --loglevel:

$ npm_config_loglevel=error node-gyp rebuild --loglevel=verbose
# v11.3.0: logger level = verbose
# v11.4.0 … main: logger level = error

• test-addon runs node-gyp rebuild --loglevel=verbose and expects gyp info ok as the last line. At error level that line is never printed, so the last line is a compiler warning.
• test-options asserts the shared logger starts at info, but earlier tests' parseArgv() calls have already set it to error from the environment.

Locally, npm_config_loglevel=error npm test on main reproduces exactly these 3 failures.

This isn't limited to CI: a non-default loglevel in .npmrc (npm exports non-default configs to scripts) silently overrides node-gyp rebuild --verbose in a package script.

Fix

• lib/node-gyp.js: remember the command-line --loglevel before the environment is read and prefer it, restoring the pre-v11.4.0 behaviour for loglevel.
  Other options keep the existing environment-last precedence (as noted in test-options.js). Changing that for e.g. --target/--nodedir could break setups that rely on npm_config_* overriding flags hard-coded in install scripts, so it's out of scope here.
• test/test-options.js: reset the shared logger before asserting its initial level, and add a test that --loglevel= and the --silly shorthand win over npm_config_loglevel, while no flag still falls back to it.

With this change, npm_config_loglevel=error npm test passes.