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

.github/workflows/ci.yml

@@ -982,6 +982,7 @@ jobs:
           - sys: mingw64
             env: x86_64
             extra_install: |
+              mingw-w64-x86_64-boost
               mingw-w64-x86_64-python-numpy
               mingw-w64-x86_64-python-scipy
               mingw-w64-x86_64-eigen3
@@ -996,7 +997,6 @@ jobs:
           mingw-w64-${{matrix.env}}-cmake
           mingw-w64-${{matrix.env}}-make
           mingw-w64-${{matrix.env}}-python-pytest
-          mingw-w64-${{matrix.env}}-boost
           mingw-w64-${{matrix.env}}-catch
           ${{ matrix.extra_install }}