fix: do not forward allow-scripts to git preparation

[carrerasdarren-cell]

Summary

• remove npm_config_allow_scripts from the environment of the nested install used to prepare git dependencies
• preserve the user-config path so the nested npm process reloads the policy from .npmrc at the correct config layer
• cover lowercase and uppercase environment keys while verifying unrelated environment and cycle-prevention state are preserved

Fixes npm/cli#9783.

Testing

• npm test -- --no-coverage
• npx eslint lib/git.js test/git.js test/fixtures/npm-mock.js
• exact offline git-dependency reproduction with npm 12.0.1: stock path fails with EALLOWSCRIPTS; patched Pacote extracts successfully

AI Disclosure

This change was written with the assistance of Codex. I reviewed the diff and tests and take responsibility for the code.