Mjcf Parser : Fix adding the sites frames #226

Job	Run time
ubuntu-latest - Env all Release	40m 30s
ubuntu-latest - Env all Debug	2m 52s
ubuntu-latest - Env all-py39 Release	1m 55s
ubuntu-latest - Env all-py39 Debug	42m 35s
macos-latest - Env all Release	1m 54s
macos-latest - Env all Debug	2m 28s
macos-latest - Env all-py39 Release	30m 11s
macos-latest - Env all-py39 Debug	17m 23s
macos-13 - Env all Release	2m 57s
macos-13 - Env all Debug	37m 22s
macos-13 - Env all-py39 Release	42m 40s
macos-13 - Env all-py39 Debug	4m 16s
windows-latest - Env all Release	1h 11m 54s
windows-latest - Env all-clang-cl Release	3m 51s
check-macos-linux-windows-pixi	1s
	5h 2m 49s

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