From the man page for pkg.conf,
ABI: string The ABI of the package you want to install. Default:
derived from the ABI of the /bin/sh binary.
it is actually
/usr/ports/ports-mgmt/pkg/work/pkg-1.13.2/libpkg/pkg_elf.c:#define _PATH_UNAME "/usr/bin/uname"
Looks like https://github.com/freebsd/pkg/commit/f75bda13dfc7aeb0f7eb44986295f45e89bf09f9#diff-287ef8c2a2b3678bf1b8a5a14e9374be fixed it upstream in February, and pkg is currently (in latest) 1.14.5, which is more recent than the version you have in your ports tree (1.13.2).