wasm build not reproducible

[leoluk]

trafficstars

wasm-pack ignores the lockfile:

https://users.rust-lang.org/t/wasm-pack-ignoring-cargo-lockfile-when-building-on-github-actions/47743

... which just broke our build:

#22 1.686 Error: Error during execution of `cargo metadata`:     Updating crates.io index
#22 1.686  Downloading crates ...
#22 1.686   Downloaded hermit-abi v0.1.20
#22 1.686 error: failed to parse manifest at `/usr/local/cargo/registry/src/github.com-1ecc6299db9ec823/hermit-abi-0.1.20/Cargo.toml`
#22 1.686 
#22 1.686 Caused by:
#22 1.686   feature `edition2021` is required
#22 1.686 
#22 1.686   The package requires the Cargo feature called `edition2021`, but that feature is not stabilized in this version of Cargo (1.55.0-nightly (d21c22870 2021-07-26)).
#22 1.686   Consider trying a more recent nightly release.
#22 1.686   See https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#edition-2021 for more information about the status of this feature.
#22 1.686 
#22 ERROR: executor failed running [/bin/sh -c cd migration && /usr/local/cargo/bin/wasm-pack build --target bundler -d bundler -- --features wasm]: exit code: 1

Their suggested fix of "just pin the versions, duh" is invalid since this still means the repo can rug us.