CI: Add a Linux job for template_debug

[akien-mga]

Seems like we've been lacking this for a while, probably assuming that target=editor + target=template_debug was enough, but it's not:

• editor: TOOLS_ENABLED + DEBUG_ENABLED
• template_debug: DEBUG_ENABLED
• template_release: neither

So to test what happens when DEBUG_ENABLED is defined but not TOOLS_ENABLED, we do need a template_debug build, the combination of the other two isn't enough.

For now I'm just quickly adding a duplicate Linux template build with Mono. We might want to review the build matrix and ensure we have good coverage on multiple platforms (but still taking care of not doubling the size of the build matrix - we try to strike a balance between coverage and efficiency, but time and energy wise).

[Repiteo]

I don't know if we'll want to keep it a separate job long-term, but this works as a stopgap solution

[fire]

A bug went through our tests. Let's catch it with a test.

[AThousandShips]

Checked and the cache limit for the debug template is appropriate so works cleanly

We might want to look into merging different workflows to reduce the matrix, been thinking about that for cases like the Android templates, and seeing what the different costs are, and seeing what takes time and not