rust: use Python JSON instead of string handling
rust: use Python JSON instead of string handling

The code previously wrote out a JSON file but used basic string handling
instead of the Python JSON library. Its cleaner to use the Python JSON
library.

Signed-off-by: Doug Goldstein <8172f793ca553050868d487fd13b508d19cae503@cardoe.com>

jmesmon/meta-rust,cardoe/meta-rust,starlab-io/meta-rust,meta-rust/meta-rust,meta-rust/meta-rust,cardoe/meta-rust,meta-rust/meta-rust,starlab-io/meta-rust,cardoe/meta-rust,jmesmon/meta-rust,cardoe/meta-rust,jmesmon/meta-rust,meta-rust/meta-rust,meta-rust/meta-rust,jmesmon/meta-rust