_compute_abi_rebuild_info: fix bug #532594

Since commit 3f0799054b4e5ef88feb59d20d262668ca79df33, there has been
a faulty variable assignment inside _compute_abi_rebuild_info which
assigns a bool value to the inst_pkg variable which is intended to
refer to a Package instance. Fix it to assign a correct value.

Fixes: 3f0799054b4e ("_compute_abi_rebuild_info: fix bug #521990")
X-Gentoo-Bug: 532594
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=532594