fix: use toFilePath in package config Uri

[Zekfad]

fixes #2231

Thanks to @d-markey #2231 (comment) for figuring it out.

This fix uses the same technique as in precompiled tests:

test/pkgs/test/test/runner/precompiled_test.dart

Line 54 in 9fbbfdb

'--packages=${(await Isolate.packageConfig)!.toFilePath()}',

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[osa1]

Thanks!