Expose a `--sdk` option to webkit-flatpak to allow roll-backs and local branch upgrades.
Created attachment 411312 [details] WIP
rollback example: `WEBKIT_FLATPAK_USER_DIR=$PWD/WebKitBuild/UserFlatpak-0-2 webkit-flatpak --branch="0.2"` Then keep WEBKIT_FLATPAK_USER_DIR set when building and running tests.