Lock the rust toolchain version

[jtran]

This has several benefits:

• Rust occasionally makes breaking changes, and locking allows us to easily git bisect. Fix build error on Rust 1.80.0 #3128 is an example.
• Consistent, reproducible builds for a given commit instead of the exact toolchain changing over time
• New toolchain versions can introduce new, non-trivial warnings which block CI until fixed. Locking prevents a rust release at an arbitrary time from block our team.
• Documents what version an arbitrary commit was intended to build on

The drawbacks are:

• We need to manually update. Dependabot doesn't currently do this. Update rust-toolchain dependabot/dependabot-core#1702
• You need to manually override the toolchain if you want to build with a different rust version than what's committed

[qa-wolf]

QA Wolf here! As you write new code it's important that your test coverage is keeping up.
Click here to request test coverage for this PR!

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Updated (UTC)
modeling-app	✅ Ready (Inspect)	Visit Preview	Aug 22, 2024 4:45pm

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.90%. Comparing base (682099c) to head (11ecf8f).
Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3617   +/-   ##
=======================================
  Coverage   87.90%   87.90%           
=======================================
  Files          67       67           
  Lines       26150    26150           
=======================================
  Hits        22986    22986           
  Misses       3164     3164           

Flag	Coverage Δ
wasm-lib	87.90% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.