CI check for wasm isn't working

[tomaka]

Building async-std for the wasm target is completely broken at the moment:

• cargo check failing on v1.6.3 tag for wasm32-unknown-unknown #851
• failed to resolve futures_timer #843

The reason why it hasn't been detected is that the CI of async-std for wasm isn't working. Here's an example.

[jbr]

#864 addresses this