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

[Sanjays2402]

Summary

When npx runs inside a global install's lifecycle script, npm_config_global=true leaks into its environment. That flag reached the Arborist instance used for the npx cache install, so arborist linked bins to the global-style <npxCache>/bin target while exec() looked for them in <cacheDir>/node_modules/.bin. The result was sh: <bin>: command not found (and, on newer trees, a broken cache entry with no package.json).

The npx cache is never a global install, so this forces global: false on the npx cache Arborist, matching the approach the issue's root-cause analysis suggested.

Fixes: #9890

Changes

• workspaces/libnpmexec/lib/index.js - force global: false on the npx cache Arborist so bins link under the cache dir's node_modules/.bin
• workspaces/libnpmexec/test/registry.js - regression test running exec with global: true against the mock registry, asserting the bin runs, no <npxCache>/bin is created, and bins land in the cache dir's node_modules/.bin

Verification

• Reproduced the broken behavior end-to-end before the fix (npm_config_global=true npm exec --yes cowsay hello failed; bins landed in _npx/bin/, no package.json written to the cache dir). After the fix the same command prints the cowsay output and bins land in <hash>/node_modules/.bin.
• New regression test fails on unfixed code (PackageJson.load ENOENT from the broken cache entry) and passes with the fix.
• Full libnpmexec suite: 9/9 suites, 119 assertions, all passing. ESLint clean on both touched files.