Improve warning when XR shaders are not enabled #238

Job	Run time
📊 Static checks / Code style, file formatting, and docs	4m 25s
🍏 iOS / Template (target=template_release)	19m 19s
🤖 Android / Template (target=template_release)	29m 15s
🏁 Windows / Editor (target=editor, tests=yes)	43m 30s
🐧 Linux / Editor w/ Mono (target=editor)	37m 40s
🏁 Windows / Template (target=template_release)	31m 9s
🍎 macOS / Editor (target=editor, tests=yes)	32m 43s
🐧 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)	38m 39s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	22m 28s
🍎 macOS / Template (target=template_release)	19m 11s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	33m 27s
🌐 Web / Template w/o threads (target=template_release, threads=no)	22m 9s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	30m 16s
🐧 Linux / Template w/ Mono (target=template_release)	24m 57s
🐧 Linux / Minimal template (target=template_release, everything disabled)	14m 36s
🪲 Godot CPP / Build and test Godot CPP	6m 53s
	6h 50m 37s

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