I bet, there's a file handle left open somewhere. This error is about CompUnit::Repository::Staging which is precompiled _three times_ in that script. Once when loading the module, the second time when installing the CORE dist and the third time to fix the source path of the installed Staging module.
It fails on the second precompilation. Though what's odd is, that the first one should write a precomp file to lib/.precomp, not to install/share/perl6/precomp. Can you please re-run with RAKUDO_MODULE_DEBUG=1?
