fix: an issue where the package path was missing when the cwd was not on the root

[lukasholzer]

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Summary

If you had a monorepo and you did cd apps/my-app and then run ntl build the PACKAGE_PATH was not set which leads to build failures for build plugins that rely on this monorepo configuration.

Now the PACKAGE_PATH constant will be set on those builds as well

Fixes https://linear.app/netlify/issue/FRA-350/enoent-no-such-file-or-directory-open-vartasknextbuild-id

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[github-actions]

📊 Benchmark results

Comparing with acafb49

• Dependency count: 1,283 (no change)
• Package size: 282 MB (no change)
• Number of ts-expect-error directives: 1,180 (no change)

[Skn0tt]

A test for this would be glamorous!

[lukasholzer]

Already on the test ;)