Update pinned nightly

[Manishearth]

From #968 (comment)

We pin nightly-2021-02-28 in CI for WASM/FFI testing so that our usage of nightly features doesn't suddenly break. We'll eventually want to bump it (especially once rust-lang/rust#85499 lands, fixing a lot of the compiler bugs plaguing yoke). It's good if we can update it to something recent.

At the moment the WASM build fails with things like

    error: WASM error: Invalid name type (at offset 18707)

..

unknown name subsection at 1187412
unknown name subsection at 1187432

Searching around this seems fixable; we should figure out what's wrong and fix it so we can use a newer nightly.

[Manishearth]

Bisected to 476acbf1e..45b3c2851 in rustc (2021-03-04 to 2021-03-05). This is quite likely due to the LLVM upgrade.

[Manishearth]

I also found WebAssembly/binaryen#1914

[Manishearth]

So there were two errors. The "invalid name type" is an error originating from the wasmparser crate. The code producing that error has since been updated to handle more cases, so I think twiggy needs to be updated to the latest wasmparser.

The other error happened in the binarysize job, haven't investigated that yet.

[Manishearth]

The twiggy bug is addressed by using rustwasm/twiggy#576 . It hasn't been released yet but should be soon. In the meantime we can install from git.