GNUInstallDirs: Fix regression on -DCMAKE_INSTALL_<dir>=<default>

In commit 42dfcbf1a5 (GNUInstallDirs: Refactor LIBDIR default
calculation, 2025-03-27, v4.1.0-rc1~384^2~2) we introduced the
`_GNUInstallDirs_LIBDIR_get_default` helper and exercised code from
commit 9789f7d05e (GNUInstallDirs: Add internal helper to compute
specific defaults, 2025-03-28, v4.1.0-rc1~384^2~3) for the first time.
Fix the latter's code to update the `CMAKE_INSTALL_<dir>` cache entry
without triggering conversion of a relative path to an absolute path.

Fixes: #27027