Test: prune old LA based on ENV variable

[jishnub]

This PR prunes the old LinearAlgebra module based on an environment variable that is set in .ci/run_tests.jl. Hopefully, this would mean that tests that are being run without this environment variable would run as expected without precompilation issues.

The goal of this PR is to ensure that when julia is being built with the master branch of LinearAlgebra, the tests would work without the need for pruning (i.e., we want to fix the test failures in JuliaLang/julia#58242).

This would also mean that to run the tests locally, one would need to set the JULIA_PRUNE_OLD_LA environment variable to true.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.74%. Comparing base (c9b6456) to head (04359c8).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1335   +/-   ##
=======================================
  Coverage   93.74%   93.74%           
=======================================
  Files          34       34           
  Lines       15752    15752           
=======================================
  Hits        14766    14766           
  Misses        986      986           

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[KristofferC]

I thought about disabling this only when we run on Julia buildkite so that includeing a test file would still work. But I am not sure if there is a good env variable to look at there?