install.mk: use INSTALL_LIB for all libraries

There's no reason to make them executable, and this throws a warning
with the Arch Linux package lint tool (namcap) for .a files.
auto merge of #5649 : thestinger/rust/lib, r=brson

There's no reason to make them executable, and this throws a warning
with the Arch Linux package lint tool (namcap) for .a files.