Add metadata overriding to the `pixi build` command.

### Problem description

I would like to be able to override the package metadata through the `pixi build` command. 

In my use case, I want to generate a conda package in CI. I would like to update the version of the package I'm building through the CLI.

```
pixi build --build-string dev_PRXXX
# OR
pixi build --version 1.2.3dev.XXX
```
