[anneal] Download and install artifacts in parallel

[mdittmer]

This change fixes #3265 by:

1. Downloading and extracting rust toolchain artifacts in parallel;
2. Setting up rust toolchain, aeneas, and lake packages in parallel with respect to each other.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.88%. Comparing base (a51752b) to head (0a36655).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3327   +/-   ##
=======================================
  Coverage   91.88%   91.88%           
=======================================
  Files          20       20           
  Lines        6076     6076           
=======================================
  Hits         5583     5583           
  Misses        493      493           

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