Debian packaging: only numeric owner/group IDs in php tarball

Recent rebuilds of src:apg have uncovered the possibility for the tarfile metadata of the php.tar.gz file in the resulting binary package to vary based on the build environment.

This changeset suggests using fixed, numeric-only owner/group identifiers when constructing the php.tar.gz tarball, so that it builds deterministically despite those build environment variances (e.g. group-id, group name).

Blocked-by #1102758 (explainer: reprotest currently passes for mainline commits, but I belive that they could and should begin failing after Rules-Requires-Root is removed. After that, reprotest should begin failing - until applying this or a similar changeset, allowing them to pass again).

Closes #1102760.