Fix `add_collision_triangles` will overwrite previously added mesh #510

Job	Run time
📊 Static checks / Code style, file formatting, and docs	1m 28s
🍏 iOS / Template (target=template_release)	10m 1s
🤖 Android / Editor (target=editor)	24m 16s
🍎 macOS / Editor (target=editor, tests=yes)	30m 1s
🐧 Linux / Editor w/ Mono (target=editor)	41m 40s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	16m 36s
🍎 macOS / Template (target=template_release)	19m 55s
🐧 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)	48m 2s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	17m 6s
🏁 Windows / Editor (target=editor, tests=yes)	52m 6s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	35m 18s
🏁 Windows / Template (target=template_release)	36m 43s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	31m 5s
🐧 Linux / Template w/ Mono (target=template_release)	25m 17s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	25m 42s
🐧 Linux / Minimal template (target=template_release, everything disabled)	14m 43s
🌐 Web / Template w/o threads (target=template_release, threads=no)	26m 21s
🪲 Godot CPP / Build and test Godot CPP	8m 26s
	7h 44m 46s

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