selftests/exec: Convert remaining /bin/sh to /bin/bash

As was intended with commit 17107429 ("selftests/exec: Perform script
checks with /bin/bash"), convert the other instance of /bin/sh to
/bin/bash. It appears that at least Debian Bookworm's /bin/sh (dash)
does not conform to POSIX's "return 127 when script not found"
requirement.

Fixes: 17107429

 ("selftests/exec: Perform script checks with /bin/bash")
Reported-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Closes: https://lore.kernel.org/lkml/02c8bf8e-1934-44ab-a886-e065b37366a7@collabora.com/


Signed-off-by: Kees Cook <keescook@chromium.org>