no way to differentiate between build-time and run-time env vars in plugins

[Thomi Richards]

As far as I can tell, BasePlugin has one method for manipulating environment variables: the env() method.

I have a custom plugin where I need to be able to set certain environment variables at build-time, so I'm setting them in this method like so:

```
    def env(self, root):
        env = super().env(root)
        env.extend([
            'CPPFLAGS="-std=c++11 -I{} $CPPFLAGS"'.format(os.path.join(
                root, 'usr', 'include')),
            'CFLAGS="-std=c11 -I{} $CFLAGS"'.format(os.path.join(
                root, 'usr', 'include')),
        ])

        return env
```

This works fine for building the snap, but I notice that these environment variables are then present in the command wrapper as well, so they're also being set for runtime. Most of the time this won't be a massive issue - setting CPPFLAGS has no effect at run-time, but it's a little surprising, and I can imagine situations where plugin authors will want more control over what's set at build time, and what's set at run-time.

[Sergio Schvezov (sergiusens)]

Can you look at the godeps plugin? That has a better mechanism for this. Then `env` method is mostly there for runtime bits only.

Keep in mind we are also going to allow parts to set `environment` and `build-environment` with those keywords allowing more flexibility from a part author point of view (and in some cases avoiding to write a new plugin).

[林博仁(Buo-ren, Lin) (buo-ren-lin)]

@sergiusens
Will `environment` and `build-environment` keys be implemented? I want to apply custom runtime command search PATH but currently `snapcraftctl` doesn't honor this.