`tc check` runs top-level prettier on subpackages' code

[jedwards1211]

Causes it to fail to respect subpackage .gitignores:

~/gh/timestream (cli) $ tc check
@jcoreio/toolchain@4.12.4
$ prettier --ignore-path .gitignore -c . '!pnpm-lock.yaml'
Checking formatting...
[warn] packages/http/coverage/lcov-report/base.css
[warn] packages/http/coverage/lcov-report/block-navigation.js
[warn] packages/http/coverage/lcov-report/decodeTimestream.ts.html
[warn] packages/http/coverage/lcov-report/FetchDataSource.ts.html
[warn] packages/http/coverage/lcov-report/fetchTimestreamData.ts.html
[warn] packages/http/coverage/lcov-report/HttpPostWritableStream.ts.html
[warn] packages/http/coverage/lcov-report/index.html