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

Fix commit 9789f7d05e (GNUInstallDirs: Add internal helper to compute
specific defaults, 2025-03-28, v4.1.0-rc1~384^2~3) to avoid updating
explicitly-specified cache entries on the first run.  Do not reset the
default value until at least the second run, during which we have
`_GNUInstallDirs_LAST_CMAKE_INSTALL_PREFIX` already defined.

Fixes: #27211
11 files changed