Bundle libsodium when building Keysmith

Currently there is an open MR to add support for encrypting token secrets to Keysmith, built on top of libsodium: https://invent.kde.org/kde/keysmith/-/merge_requests/33 Once that MR (or something like it) is merged in, Keysmith builds will start to depend on libsodium. This MR attempts make building libsodium part of building Keysmith on Binary Factory.

The long shell string is based on the example of building freetype for libpoppler at: https://invent.kde.org/sysadmin/ci-tooling/-/blob/master/system-images/android/sdk/build-poppler#L14 It may or may not be better to extract the rather convoluted shell blob to its own builder helper.