fix(installer): isolate system curl from AppImage library paths

[nekomario28]

Summary

• keep AppImage-bundled libraries available to the application itself;
• isolate the installer’s system curl subprocess from AppImage library-path injection;
• add focused regression coverage for the installer environment boundary.

This is a narrow defense-in-depth fix for #57 rather than an AppImage-wide RUNPATH redesign.

Validation

• exact candidate: 28005c80f221a3d7f289993e6035bb1a0c163919;
• CachyOS rolling host: installer regression path passed with system curl;
• tests/install/test_install_script.py: 3 passed;
• earlier fork Ubuntu 24.04 validation: green;
• current-upstream merge/drift checks were clean at the publication fence.

Boundary

The exact affected Fedora/Bazzite AppImage end-to-end path has not been re-run here. A focused test from an affected reporter would be useful before promotion out of Draft.

Fixes #57.