distutils-r1.eclass: Pass --skip-build when installing

distutils-r1.eclass: Pass --skip-build when installing

Explicitly pass --skip-build in order to prevent distutils from
rebuilding files when installing.  This is especially relevant to some
packages that build extensions, and the extensions get built again
in src_install().

Technically, this can break some customized build systems that write
implementation-specific data into source directory, and right now
the implied rebuild caused it to be rewritten when installing.  With
this patch, the newest version of the file would be installed for all
interpreters.  I am not aware of any such packages, though.

Signed-off-by: Michał Górny <mgorny@gentoo.org>