Dogfood `tinyexec` in local Vitest

[AriPerkkio]

Ref. #90 (comment), cc. @43081j

In Tinypool we used to override local Vitest's tinypool version to the local ./dist one, so that all unpublished changes would check Vitest compatibility immediately.

 "pnpm": {
   "overrides": {
     "vitest>tinypool": "link:./"
   }
 }

https://github.com/tinylibs/tinypool/blob/40b4b3eb926dabfbfd3d0a7e3d1222d4dd1c0d2d/package.json#L60-L64

We should do the same with tinyexec. I quickly tested this locally with pnpm, and it does catch #90 when reverted. I wasn't able to make this work with plain npm, so switching to pnpm might be needed.

[AriPerkkio]

Decided not to: #94 (comment)