Fix TestEnv for subpackages: precompile only the new test Project, take 2

[Drvi]

Like #90, but uses Pkg._auto_precompile instead of Pkg.precompile to respect the JULIA_PKG_PRECOMPILE_AUTO env var.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.27%. Comparing base (09933b2) to head (dcc3ee3).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #114      +/-   ##
==========================================
+ Coverage   78.17%   78.27%   +0.09%     
==========================================
  Files          34       34              
  Lines         921      925       +4     
==========================================
+ Hits          720      724       +4     
  Misses        201      201              

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[NHDaly]

Fantastic. LGTM - thanks @Drvi great fix!

[NHDaly]

double checking this in our internal tests one more time

[Drvi]

Internal test succeeded, so I'm going to merge this. Thanks!