FindEXPAT: Implement EXPAT_USE_STATIC_LIBS for non-Windows platforms

In commit 302501ad36 (FindEXPAT: add a EXPAT_USE_STATIC_LIBS hint,
2023-06-19, v3.28.0-rc1~447^2) this flag was introduced but was only
implemented for windows like systems. To make the flag complete and also
behave as you would expect on non-windows systems cmake also looks for
`.a` files now.

Fixes: #26177