community/olm: upgrade to 3.2.12

https://gitlab.matrix.org/matrix-org/olm/-/tags/3.2.12

This release includes the following changes since 3.2.11:

• Expose olm_sas_calculate_mac_fixed_base64 in the bindings.
• Allow memory to grow in wasm. Thanks to benkuly for the suggestion.
• Fix Python type hints.
• Some Python build fixes.
• Initial work on a Nix flake for building and testing.