FindPkgConfig: Fix preservation of ENV{PKG_CONFIG_ALLOW_SYSTEM_LIBS}

Fix the ENV save/restore added by commit 36979f5b43 (FindPkgConfig:
Tolerate PKG_CONFIG_SYSTEM_LIBRARY_PATH in environment, 2021-07-13,
v3.22.0-rc1~407^2) to unset the variable if it was originally not set.

While at it, clarify the value to which we temporarily set it. The
`pkg-config` and `pkgconf` tools only check that the variable is set,
and do not care about the value.  Set it to a more true-looking value.

Issue: #22148