PVF: Incorporate wasmtime version in worker version checks

[mrcnski]

For example, say someone downloads the node and workers at a release version, and then switches to master for some bug fixes. He could be running an updated node without updating the workers, but the node version hasn't changed so the version check passes. But wasmtime may have been bumped since then.

Wasmtime bumps are rare enough, that we can enforce rebuilds without impacting the dev UX. Should be simple to implement; see the new SUBSTRATE_WASMTIME_VERSION env var.

Originally posted by @mrcnski in #1918 (comment)

[s0me0ne-unkn0wn]

Closing as superseded by #3139