apkbuild-lint: add CUSTOM_VALID_OPTIONS

Allow to export the variable with APKBUILD options, that may only be relevant in postmarketOS and possibly other projects around Alpine.

• Related: https://wiki.postmarketos.org/wiki/Build_internals#pmbootstrap_specific_APKBUILD_options
• Related: https://gitlab.com/postmarketOS/pmaports/-/issues/553