scripts/sanitizer-env.sh: check and warn if OUR_PRELOAD is not a file

scripts/sanitizer-env.sh: check and warn if OUR_PRELOAD is not a file

As seen with the upcoming clang 19 change, the library name has changed.
Enhance the current test to check if the compiler provided string is a
file and produce a warning with some handy tips when that's not the
case.

v2:
 - convert to heredoc
 - output to stderr

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/219
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>