Allow detecting when building as an engine module #19003

Job	Run time
📊 Static checks / Code style, file formatting, and docs	34s
🍏 iOS / Template (target=template_release)	12m 30s
🤖 Android / Template (target=template_release)	11m 46s
🐧 Linux / Editor w/ Mono (target=editor)	12m 43s
🌐 Web / Template (target=template_release)	7m 20s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	28m 5s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	13m 25s
🍎 macOS / Editor (target=editor, tests=yes)	16m 40s
🏁 Windows / Editor (target=editor, tests=yes)	16m 41s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	13m 51s
🍎 macOS / Template (target=template_release)	9m 46s
🏁 Windows / Template (target=template_release)	14m 4s
🐧 Linux / Template w/ Mono (target=template_release)	10m 16s
🐧 Linux / Minimal template (target=template_release, everything disabled)	2m 20s
🪲 Godot CPP / Build and test Godot CPP	6m 53s
	2h 56m 54s

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