Change the `govet.sh` script so that it runs the shadows check the same way for Go 1.12 and Go 1.13 builds, wrapping the shadow analyzer in a tiny main function that catches and ignores the `unsafeptr` flag that gets passed through by `go vet` in Go 1.13+.

On a couple occasions we've had builds break after #1778 was merged. The failures reported have only been on Go 1.13. The specific failure seems to be inconsistent and was reported to golang/go#34557. The fact that we've seen this failure only occur on Go 1.13 is curious. It's possible that there's a change in the language that is contributing to that behavior, or it's possible that different way we run the shadow checker for our 1.13 builds is causing tests to be included in some different way.

When I implemented #1778 I had the option of making the 1.12 and 1.13 builds execute the binary the same way like in this PR, or to execute differently like how it was done in #1778. I mostly flipped a coin.

I'm not sure if doing it this other way will yield more consistent go vet runs, but I figure worth a shot before we remove it.

Known limitations & issues:
- This might not make a difference to the failures.
- This doesn't address the actual cause of the failures, or the inconsistency, and is just attempting to make our own two builds more consistent with each other.