Correct specification for deprecated CNB_STACK_ID

Restores the specification for `CNB_STACK_ID` to the definition that was used prior to the docs refactor in buildpacks#335.

Fixes buildpacks#396.
Backport of buildpacks#397.

Signed-off-by: Ed Morley <501702+edmorley@users.noreply.github.com>

platform.md

@@ -1570,10 +1570,13 @@ _Deprecated in Platform API 0.12._
 
 For compatibility with older platforms and older buildpacks, base image authors SHOULD ensure for build images and run images:
 
-- The image config's `Env` field has the environment variable `CNB_STACK_ID` set to the stack ID.
 - The image config's `Label` field has the label `io.buildpacks.stack.id` set to the stack ID.
 - The image config's `Label` field has the label `io.buildpacks.stack.mixins` set to a JSON array containing mixin names for each mixin applied to the image.
 
+And for build images:
+
+- The image config's `Env` field has the environment variable `CNB_STACK_ID` set to the stack ID.
+
 Where `CNB_STACK_ID` SHALL be directly inherited by buildpacks without modification.
 
 To upgrade, the platform SHOULD upgrade all buildpacks to use Buildpack API `0.10` or greater.