Update wasi dependency

[mati865]

cc rust-lang/rust#63806
It failed with

invalid license Apache-2.0 WITH LLVM-exception in /checkout/obj/build/tmp/distcheck/src/../vendor/wasi/Cargo.toml

It could be worked around by adding exception for wasi but since it's license has been changed more optimal solution is to just update wasi.

[newpavlov]

There is already wasi v0.7. See #100. ;)

[josephlr]

Closing in favor of #100. Thanks @mati865 for the fix!