Allow expressing `meta` and `env` blocks as map attributes as well.
`env` and `meta` should support arbitrary key and values, yet hcl2
restricts the keys to valid identifiers. For example, block attribute
identifiers may not contain dots, `.`, which frequently used in meta
fields, and sometimes in environment variable fields.

This change attempts to parse `env`/`meta` both as an attribute and as a
block.

This additionally allows better expressivity for env/meta blocks, using
functions. For example, one can reuse a set of environment variables for
multiple tasks, using a local common_envs value:

```hcl
env = merge(local.common_envs, {"more_env_key", "..."})
```