-s flag fails in tools=no builds without project

[leonkrause]

Godot version:
Tried 3.0.2 and 928cdb4

OS/device including version:
Tried HTML5 and Windows

Issue description:
Attempted to run godot -s script.gd with godot being a template executable (tools=no).
Neither the working directory, nor any of its parents, contain a project. No project is specified on the command line.
Expected script.gd to run as main loop with default values used for project settings.
Instead exited right away after logging the following:

Error: Could not load game path '.'.
**ERROR**: Condition ' !_start_success ' is true. returned: false

Source:

godot/main/main.cpp

Line 1230 in 928cdb4

ERR_FAIL_COND_V(!_start_success, false);

Steps to reproduce:
Run godot -s script.gd with godot being a template executable.

Minimal reproduction project:

extends MainLoop

func _iteration(dt):
    OS.alert('Success: main loop script is running')
    return true

[Xrayez]

While working on #41190, I suspect the issue may be in ClassDB::can_instance() method which behaves differently in exported projects.

If someone wants to workaround this currently, you may as well consider using the feature added in #41190 because it would work even in export templates, unless you need to supply different main loop scripts of course.