ix scrubs environment when it shouldn't when going through aa-exec

Somewhere between 3.13 and 4.4, the scrubbing behavior of ix for aa-exec changed. Non-aa-exec cases work correctly everywhere (no scrubbing). For example, on Ubuntu 12.04 and 14.04 we have:

* ux does not scrub
* Ux does scrub
* ix does not scrub

but in 16.04 and later we have:

* ux does not scrub
* Ux does scrub
* ix does scrub # WRONG

I discussed this with jjohansen some time ago (just now filing the bug) and we concluded that ix shouldn't scrub and the behavior change for aa-exec with ix was unintentional, but that this needed to be investigated.

Attached is a reproducer:

$ tar -zxvf ./reproducer.tar.gz
reproducer/
reproducer/test.sh
reproducer/driver.sh
reproducer/profile

$ cd reproducer && ./driver.sh
Loading apparmor profiles...
...

ix should scrub: FAIL: ix scrubs
Ux should scrub: PASS
ux should not scrub: PASS

FAIL
[1]

The separate reproducer is:

$ cat ./profile
#include <tunables/global>

profile aaexec-ix {
  #include <abstractions/base>
  #include <abstractions/bash>
  #include <abstractions/perl>

  /bin/dash ixr,
  /bin/grep ixr,
  /**/test.sh r,

  @{PROC}/*/attr/exec rw,
  change_profile -> unconfined,

  /usr/{,s}bin/aa-exec ixr,
}

$ cat ./test.sh
#!/bin/sh
set -e

export LD_LIBRARY_PATH="foo"
aa-exec -p unconfined -- /bin/dash -c 'env' | grep LD_

$ sudo apparmor_parser -r ./profile
$ export LD_LIBRARY_PATH=foo

Then on (at least) 4.4 and higher:
$ aa-exec -p aaexec-ix -- ./test.sh | grep foo
[1]
$

and on (at least) 3.13 and below:
$ aa-exec -p aaexec-ix -- ./test.sh | grep foo
LD_LIBRARY_PATH=foo
$

Note: I also tested the perl aa-exec on newer releases and it shows the same ix scrubbing behavior as the binutils aa-exec.