cmd/go/internal/work/exec: throw an error when buildP is negative

[Yuuki77]

Fixed a problem where an error would not occur
when a negative value was specified for the p flag.

go build -p=0
now should throw an error.

this is my first pr to this project.
If there's anything I'm missing, please let me know 🙏
Fixes #46686

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[Yuuki77]

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