CPack/DEB: Fix regression when package architecture is explicitly specified

Since commit 302a645d73 (CPack/DEB: use `triggers` instead of legacy
postXXX scripts for `ldconfig`, 2026-03-05, v4.4.0-rc1~583^2) the
`triggers` feature refers to `DPKG_EXECUTABLE` if the latter is defined.
Unfortunately, it is defined only if one particular execution branch has
executed.  Move the `find_program(dpkg)` to the function scope to make
it available in all execution branches.