Fix: --db-prefix silently drops following CLI flags (#9042)
`--db-prefix` used to be defined with an optional value (`::`), but PHP's getopt() only accepts a value for an optional-value long option via `--db-prefix=value`, never the documented `--db-prefix value` form.
Passing it space-separated leaves the value as an unrecognized argument and silently drops every flag that follows (e.g. --disable_update) from getopt()'s result.
Revert to a somewhat required value (`:`), which still allows an empty prefix via `--db-prefix ''`.
Closes https://github.com/NixOS/nixpkgs/issues/530367
Changes proposed in this pull request:
- deprecate `--db-prefix=test_` (nowhere documented this way) and allow `--db-prefix test_`