[now dev] Add test case for lazily installing builders

[TooTallNate]

This is a follow up for #2509 to add a unit/regression test ensuring correct functionality.

[codecov-io]

Codecov Report

Merging #2510 into canary will decrease coverage by 0.41%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           canary    #2510      +/-   ##
==========================================
- Coverage   12.35%   11.94%   -0.42%     
==========================================
  Files         257      257              
  Lines        9359     9304      -55     
  Branches     1062     1046      -16     
==========================================
- Hits         1156     1111      -45     
+ Misses       8084     8072      -12     
- Partials      119      121       +2

Impacted Files	Coverage Δ
src/util/dev/builder-cache.ts	68.85% <71.42%> (-2.33%)	⬇️
src/util/metrics.ts	68.18% <0%> (-11.13%)	⬇️

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