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

As was intended with commit 17107429947b ("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: 17107429947b ("selftests/exec: Perform script checks with /bin/bash")
Reported-by: Muhammad Usama Anjum <[e-mail]>
Closes: https://lore.kernel.org/lkml/[e-mail]/
Signed-off-by: Kees Cook <[e-mail]>

tools/testing/selftests/exec/Makefile

···
 
 $(OUTPUT)/subdir:
 	mkdir -p $@
-$(OUTPUT)/script:
-	echo '#!/bin/sh' > $@
+$(OUTPUT)/script: Makefile
+	echo '#!/bin/bash' > $@
 	echo 'exit $$*' >> $@
 	chmod +x $@
 $(OUTPUT)/execveat.symlink: $(OUTPUT)/execveat