scripts: bundle-libraries.sh: retain preloaded libraries

scripts: bundle-libraries.sh: retain preloaded libraries

Since the introduction of fakeroot support, wrapped SDK executables might
be invoked from a shell that has libfakeroot.so preloaded.

Since we're using preloading as well in order to mangle argv[0] when
invoking the shipped ELF interpreter directly, we must take care of
preloading the already preloaded libraries as well, to avoid invoked
programs losing their fakeroot capabilities.

Extend the bundle-libraries.sh script to take any existing $LD_PRELOAD
into account when invoking the target ELF executable with a preloaded
runas.so library.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>