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

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

Change-Id: I3b19773ef095fad0e0419100d317727c2268699a
GitHub-Last-Rev: e5c57804d9995f5c858aa42d9de21b25de246eb5
GitHub-Pull-Request: golang/go#47360
Reviewed-on: https://go-review.googlesource.com/c/go/+/336751


Reviewed-by: Jay Conrod <jayconrod@google.com>
Trust: Jay Conrod <jayconrod@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>