We need the face target here, not the whole texture target.
This fixes the one case that was actually broken, and avoids hitting
an assert in debug swiftshader for all tests in this group.

Bug: b/141916742
Test: dEQP-EGL.functional.image.api.create_image_gles2_cubemap*
Change-Id: I25ef8f6d6e2909adfc646e68f1711589777017ba
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/36948Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>