ci: Fix handling of $PKG_CONFIG_LIBDIR

There are two environment variables that are baked into our
cross-compilation container images at build time, $CONFIGURE_OPTS
and $PKG_CONFIG_LIBDIR: the former contain the options necessary
to convince configure to perform a cross build rather than a
native one, and the latter is necessary so that pkg-config will
locate the .pc files for MinGW libraries. Container images that
are not intended for cross-compilation will not have either one
defined.

The problem is that, while an empty $CONFIGURE_OPTS is completely
harmless, setting $PKG_CONFIG_LIBDIR to an emtpy value will
result in pkg-config not looking in its default search path, thus
not finding any library, and subsequently breaking native builds.

To work around this issue, only pass $PKG_CONFIG_LIBDIR to sudo
when the value is set in the calling environment.

Fixes: 71517ae4db35c4dcc6c358d60d3a6d5da0615d39
Signed-off-by: Andrea Bolognani <abologna@redhat.com>