meson: correctly specify the dependency relationship for playtests

meson: correctly specify the dependency relationship for playtests

It depends on the zstd program being built, and passes it as an env
variable. Just like datagen. But for datagen, we explicitly depend on
it, while for zstd, we assume it's built as part of "all".

This can be wrong in two cases:
- when running individual tests, meson can (re)build just what is needed
  for that one test
- a later patch will handle building zstd but not by default