Fix benchmarks (after 1.58.1 update)

[maurolacy]

Required for edition2021 feature. I'm now getting the following error in CI:

  ...
  Downloaded uint v0.9.2
error: failed to compile `cosmwasm-vm v1.0.0-beta4`, intermediate artifacts can be found at `/tmp/cargo-installFy1UUa`

Caused by:
  failed to parse manifest at `/usr/local/cargo/registry/src/github.com-1ecc6299db9ec823/uint-0.9.2/Cargo.toml`

Caused by:
  feature `edition2021` is required

  this Cargo does not support nightly features, but if you
  switch to nightly channel you can add
  `cargo-features = ["edition2021"]` to enable this feature
  ...

Swtching to current stable seems to be the simplest / stablest way to solve this.

Update: Parts of this PR were sneakily merged in #462. The remaining here is for fixing the benchmarks in CI, which run on main and broke after merging #462.