Running `--doctool` wrongly creates `shader_cache` directory in working directory

[akien-mga]

Godot version:
master (896aa94)

OS/device including version:
Mageia 9

Issue description:
When running --doctool, a shader_cache repository is created in the working directory, since the addition of shader caching in #49050.

Normally running the editor should generate and use the shader_cache in the editor config dir, but --doctool is badly set up in main.cpp as neither really editor nor project manager nor standalone project, so the logic seems to fail here.

This is similar to #41435 and would likely be fixed the same way (both bugs might be the same).

Steps to reproduce:

• godot --doctool
• ls