Broken if `/tmp` is `noexec` on Unix

[gaborcsardi]

> pak::pak("filelock")
Error in dyn.load(dll_file) :
  unable to load shared object '/tmp/RtmpX1naxg/filebf81402596c3/glue/libs//glue.so':
  /tmp/RtmpX1naxg/filebf81402596c3/glue/libs//glue.so: failed to map segment from shared object

The same bug as https://github.com/r-lib/callr/issues/192, essentially. We'll need to fall back to loading the libs from the private lib, without making a copy.