chore: Specify execa devDep

[rschristian]

execa is used as part of the test suite, and while not specifying it doesn't immediately cause problems here, it is tripping up the ecosystem-ci I'm writing. This seemed like a reasonable enough of a thing to correct, however, as right now the test suite is relying on transient copies of execa to be lying around for use.

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Essentially, as execa is not specified as a dependency, pnpm is able to resolve it from other places outside of the prefresh project (depending on directory structure and whatnot). execa is a Sindre package that went ESM-only in v6, so pulling anything newer obviously chokes up the require()s used here. Explicitly specifying it corrects pnpm's resolution, ensuring we only use a compatible v5.

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