abuild: warn when files are installed to non-usr locations

As agreed upon by the TSC: https://gitlab.alpinelinux.org/alpine/tsc/-/blob/master/minutes/2024-08-15.md?ref_type=heads#implement-usr-merge-73 Should be changed to an error once some core packages are adjusted Alpine 3.21 is released.

CC @calebccff @pabloyoyoista