fix(exec): don't inherit global config when installing to the npx cache

[lazerg]

What / Why

An npx spawned from a lifecycle script during npm install -g inherits npm_config_global=true from the script environment, so exec() builds the npx cache Arborist with global: true. On reify the installed package counts as a global top-level install, and bin-links puts the symlinks in <npxCache>/bin rather than <installDir>/node_modules/.bin, which is the only path exec() adds to binPaths. A warm npx cache hides the problem because nothing is reified.

On npm 10.x that surfaces the way the issue reports it, as command not found and a rollback of the whole global install. Since npm 11.2 the failure comes a step earlier: global mode also skips saving the ideal tree, so the PackageJson.load(installDir) after reify throws Could not read package.json ... npxCache/<hash>/package.json. Same cause either way.

The npx cache is never a global install, so its Arborist now sets global: false explicitly.

Testing

New case in workspaces/libnpmexec/test/registry.js that runs exec with global: true and checks the bins end up in the cache entry's node_modules/.bin and not at the cache root. It fails on the current code and passes with the fix.

Also checked by hand with a cold cache: npm_config_global=true node . exec --yes -- cowsay hello errors on latest and works after the change.

References

Fixes #9890