Escape &,> in --set and --build-arg

Feature Request: Escape the `&` `>` passed in `--build-arg` and `--set` options, eg

```
acb build --build-arg arg1=a&b => arg1=a\&b
acb exec --set key1=a>b => key1=a\>b
```

Maybe it's just safe to escape all chars passed in `--build-arg` and `--set`, eg `\a\r\g\1\=\a\&\b`

Start by locating the argument parsing and handling for the `--build-arg` and `--set` options used by `acb build` and `acb exec`. Done means values containing `&` and `>` are escaped as shown; the broader question of whether all characters should be escaped remains part of the issue.

Written by the indexing model from the issue text.