Install version ^0.12 of wasm-pack

[Woutifier]

Version 0.13 introduces a breaking change by adding a dependency on cmake. We'll automatically install that version when running this and wasm-pack isn't yet installed in the environment.

Closes #589

[kflansburg]

I've thought a bit about this and I'm a little concerned with pinning this. We've had trouble in the past with not keeping pinned dependencies up to date. Also this code is kind of hacky in the first place in that it is attempting to detect and install a system-wide CLI tool.

Would it make sense to take wasm-pack instead as a crate dependency and invoke the Build object directly? https://github.com/rustwasm/wasm-pack/blob/master/src/command/build.rs#L210

[nulltrope]

Would it make sense to take wasm-pack instead as a crate dependency and invoke the Build object directly? https://github.com/rustwasm/wasm-pack/blob/master/src/command/build.rs#L210

(chiming in as I reported the linked issue) adding wasm-pack as an actual crate dependency & invoking directly sounds cleaner, and won't sidestep the regular cargo dep system.

Thankfully an update was pushed today to wasm-pack v0.13.0 that removes the dependency on cmake, so even folks pinned to an earlier version of workers-rs should be unblocked now.

[Woutifier]

Also agreed, closing this in favor of #592