fix: automatic file: protocol handling

[dominikg]

only do it for existing directories and use relative path

• use more specific local directory check and make sure the path is absolute
• use pnpm 7.18.1 for ecosystem-ci itself
• change packageManager constraint in package.json of test project if it would use pnpm 7.18.0

[dominikg]

see pnpm/pnpm#5754. if that does not get resolved quickly, we'll have to figure out how to enforce staying on 7.17

[dominikg]

updated the implementation here to avoid pnpm 7.18.0 , see https://github.com/vitejs/vite-ecosystem-ci/actions/runs/3629527374/jobs/6121834438#step:8:406