Re-apply JULIA_LOAD_PATH leakage fix when running as a Pkg app

[gbaraldi]

Summary

Re-applies the fix from #127 (commit c3716ef, reverted in edd1e25) and fixes the test failure that prompted the revert. Closes #124, fixes #106.

When JuliaC is installed as a Pkg app, the generated shim exports JULIA_LOAD_PATH=<JuliaC project dir>. The compile subprocesses JuliaC spawns (Pkg.instantiate/Pkg.precompile and juliac-buildscript.jl) inherit it, locking the load path to JuliaC's own project and breaking the user project's precompilation: Missing source file for Base.PkgId(...).

3a17f5c (#59) originally fixed this with an unconditional JULIA_LOAD_PATH => nothing override; 6940a31 (#96) inadvertently regressed it by making the override conditional on is_trim_enabled.

Fix

• Always clear JULIA_LOAD_PATH for the spawned subprocesses (restores 3a17f5c).
• For --trim, inject HostCPUFeatures.freeze_cpu_target=true directly into the temp project copy's LocalPreferences.toml instead of constructing an ad-hoc env on JULIA_LOAD_PATH — separates environment isolation from preference injection.
• Adds TOML (stdlib) to [deps]/[compat] for the preferences injection.

Why #127 was reverted, and what's different here

#127's regression test (Pkg app JULIA_LOAD_PATH isolation) simulated the shim by manually setting JULIA_LOAD_PATH=$ROOT/. That works in a dev checkout because the dev tree has a resolved Manifest.toml, but *Manifest*.toml is in this repo's .gitignore. When Julia's CI installs JuliaC via Pkg.add, no Manifest.toml ends up under $ROOT, so the simulation couldn't even load JuliaC's own deps in the subprocess and failed before exercising the fix.

This PR keeps both regression tests but reorganises them:

• Project with dependencies (no trim) (#124) / (trim) (#124) — programmatic API tests, always on, would have caught "Missing source file" for direct dependencies introduced in v0.3.1 #124 before merge.
• Pkg app end-to-end (#106) — uses Pkg.Apps.develop to install JuliaC and run its real shim against a user project (Unix only). Always on.
• Pkg app JULIA_LOAD_PATH isolation (#106) — the simulated-shim variant, gated on JULIAC_TEST_PKG_APP_SHIM_SIM=1 with a hard precondition check on $ROOT/Manifest.toml. JuliaC's own CI sets the env var in .github/workflows/ci.yml; downstream consumers (like Julia's CI) won't run it.

Test plan

• Reproduced "Missing source file" for direct dependencies introduced in v0.3.1 #124 on main with a minimal SHA-using project under JULIA_LOAD_PATH=$JC/
• Verified the fix resolves the repro in both trim and no-trim paths
• End-to-end check: Pkg.Apps.develop + invoke the generated shim → executable runs and prints expected output
• Full Pkg.test() passes locally with JULIAC_TEST_PKG_APP_SHIM_SIM=1
• CI on this PR

🤖 Generated with Claude Code

[gbaraldi]

Checked on upstream CI JuliaLang/julia#61840. And it no longer breaks