Give organize access to variables set during the build step

[dilyn-corner]

What needs to get done

When snapcraft performs the build step for a part, keys in build-environment or variables set during the build should exist for the organize portion of the build step to use.

Why it needs to get done

Suppose something like the below:

parts:
  foo:
    build-environment:
      NAME: bar
      VERSION: baz

If some piece of foo needs to be in a specifically named directory, this must currently be done manually:

    override-build: |
      craftctl default
      mv -f "${CRAFT_PART_INSTALL}/my/file" "${CRAFT_PART_INSTALL}/$NAME/$VERSION/my/file"

The organize part of the build step is a more natural use-case:

    organize:
      my/file: $NAME/$VERSION/my/file

The utility becomes more obvious just in case $NAME or $VERSION are generated and only known during the build (i.e. in the case of kernel snaps):

    override-build: |
      craftctl default

      kver="$(basename "${CRAFT_PART_INSTALL}/lib/modules/"*)"
    organize:
      modules/**/kernel/drivers/net/wireless/rtw88: (component/wlan)/modules/$kver/kernel/drivers/net/wireless/rtw88