Add a feature to disable update-checking

[alexcrichton]

If this is packaged elsewhere the update-check will want to be disabled!

[joshtriplett]

Thanks! I wrote some thoughts on what this could look like in #23 (comment) , with the aim of keeping the prebuilt binaries just as they are while making installs of cargo-wasi-src closer to what distributions and users of vendoring would need.

[alexcrichton]

Sounds resonable to me! I'm down for disabling this feature by default and only enabling it in the prebuilt binaries.

[joshtriplett]

@alexcrichton Awesome, thank you!