selftests/exec: Perform script checks with /bin/bash

It seems some shells linked to /bin/sh don't have consistent behavior
with error codes on execution failures. Explicitly use /bin/bash so that
"not found" errors are correctly generated. Repeating the comment from
the test:

/*
 * Execute as a long pathname relative to "/".  If this is a script,
 * the interpreter will launch but fail to open the script because its
 * name ("/dev/fd/5/xxx....") is bigger than PATH_MAX.
 *
 * The failure code is usually 127 (POSIX: "If a command is not found,
 * the exit status shall be 127."), but some systems give 126 (POSIX:
 * "If the command name is found, but it is not an executable utility,
 * the exit status shall be 126."), so allow either.
 */

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>
---
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: linux-mm@kvack.org
Cc: linux-kselftest@vger.kernel.org