ci: drop boost from 32-bit mingw32 install (#6091)

* ci: drop boost from 32-bit mingw32 install

MSYS2 removed the mingw-w64-i686-boost package (32-bit i686 is being
phased out), so the mingw32 matrix entry now fails at the pacman install
step with "target not found: mingw-w64-i686-boost". Boost is optional
test coverage (boost::optional / boost::variant casters, gated behind
PYBIND11_TEST_BOOST); without it those tests are skipped and the rest of
the job builds and runs normally. Move boost to the mingw64 extra_install
so the 64-bit job keeps that coverage.

Assisted-by: ClaudeCode:claude-opus-4.8

* Apply suggestion from @henryiii