Set JULIA_PROJECT when running the subprocess in ensurecompiled()

[DilumAluthge]

Suppose that you have two different projects, where one project contains the packages that will go into the sysimage, and the other project is a "build project".

As an example, consider the examples/MyLib folder in this repo:

• The examples/MyLib/Project.toml is the "main project" that contains the packages that will go into the sysimage.
• The examples/MyLib/build/Project.toml is the "build project". The packages in the "build project" will not go into the sysimage. In this example, the "build project" contains Libdl and PackageCompiler, but neither of those will go into the sysimage.

When the ensurecompiled() function is run, we want to make sure that it precompiles the "main project", not the "build project". That's what this PR does.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.58%. Comparing base (221968a) to head (b38c01c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #995      +/-   ##
==========================================
+ Coverage   84.56%   84.58%   +0.01%     
==========================================
  Files           3        3              
  Lines         823      824       +1     
==========================================
+ Hits          696      697       +1     
  Misses        127      127              

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[KristofferC]

I would have think this is already the case from the existing code? Do you have something like an MWE where it isn't?

julia> Base.active_project()
"/home/kc/JuliaPkgs/PackageCompiler.jl/examples/MyLib/build/Project.toml"

julia> project = ".."
".."

julia> run(addenv(`$(Base.julia_cmd()) -e '@show Base.load_path()'`, "JULIA_LOAD_PATH" => "$project:@stdlib"));
Base.load_path() = ["/home/kc/JuliaPkgs/PackageCompiler.jl/examples/MyLib/Project.toml", "/home/kc/.julia/juliaup/julia-1.10.6+0.x64.linux.gnu/share/julia/stdlib/v1.10"]