the install process attempts to unlink /usr/lib/libGL.so even if NothingOutsideProjectRoot is set. it's not really okay to let this fail silently though, because under a normal install process this should get removed. the real fix is to build libGL from Mesa instead of Xorg, and _always_ install it to /usr/lib like the ABI requires.
i have no intention of fixing this in 6.9, and it's irrelevant in 7.0 and later.
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