The implicit interpreter is dependent on the environment, and isn't
guaranteed to be /bin/sh. Some packagers call this script directly, and
in certain environments such as containers using qemu-user through
binfmt_misc emulation on Linux it doesn't fallback to /bin/sh.

To fix these cases we add the interpreter explicitly.
Signed-off-by: J. Dekker <jdek@itanimul.li>