npm 10 hides postinstall output: install-time nudges never print

Tested on npm 10.9.9 / Node 22: a package's postinstall script runs on npm i -g ./pkg.tgz, but its console.log AND console.error output is not shown; the terminal only says 'changed 1 package'. It appears only with --foreground-scripts, or when the script exits non-zero, in which case it is printed as npm error lines and the install fails. So a 'next steps' message printed from postinstall is invisible to almost every user. Inside the script, npm_config_global is "true" for -g installs and unset when the package is installed as a dependency. If the message matters, print it from the CLI on first run instead.