Allow end users to preprocess default environment variables before
injecting them into container using `--env-merge`
Usage
```
podman run -it --rm --env-merge some=${some}-edit --env-merge
some2=${some2}-edit2 myimage sh
```
Closes: https://github.com/containers/podman/issues/15288
Signed-off-by: Aditya R <arajan@redhat.com>
Aditya R