fix: avoid conflicting prefer config flags

[pupuking723]

Fixes #472.

FetcherBase currently always forwards both --prefer-offline=false and --prefer-online=false to the inner npm install command used while preparing git dependencies. Newer npm treats these configs as mutually exclusive, so setting both keys, even to false, can trip the stricter validation.

This changes the generated npm CLI config so the prefer flags are only forwarded when they are enabled:

• preferOffline: true adds --prefer-offline=true
• preferOnline: true adds --prefer-online=true
• the default path omits both prefer flags

The existing snapshot for the default config was updated, and the fetcher test now asserts the enabled/disabled cases explicitly.

Verification:

• npx tap test/fetcher.js
  • Relevant assertions passed.
  • The command still exits non-zero when run as a single file because the repo's global coverage threshold expects the full suite.
• npm run eslint -- lib/fetcher.js test/fetcher.js
• git diff --check

I also ran npm test; it progressed through the relevant test/fetcher.js suite successfully, then stopped later in test/git.js during local fixture setup with ENOENT for test/tap-testdir-git/submodule-repo/package.json, which appears unrelated to this change.