flatpak-cargo-builder: unwrap tomldoc

[theoden8]

this was generating invalid hashes because tomlkit.load and tomlkit.parse return TomlDocument instead of dict.

[bbhtt]

Where was it producing wrong hashes?

[bbhtt]

also prefix the commit message with cargo:

[theoden8]

...
- type: archive
  archive-type: tar-gzip
  url: https://static.crates.io/crates/zstd-sys/zstd-sys-2.0.15+zstd.1.5.7.crate
  sha256: !!python/object/apply:tomlkit.items.String
  - *id001
  - eb81183ddd97d0c74cedf1d50d85c8d08c1b8b68ee863bdee9e706eedba1a237
  - eb81183ddd97d0c74cedf1d50d85c8d08c1b8b68ee863bdee9e706eedba1a237
  - !!python/object:tomlkit.items.Trivia
    indent: ''
    comment_ws: ''
    comment: ''
    trail: '

      '
  dest: cargo/vendor/zstd-sys-2.0.15+zstd.1.5.7
...

it wasn't serializing right

[bbhtt]

Ah right, I forgot about the YAML output.