Build and check source distributions and wheels independently in build packages

### Housekeeping

- [X] I am a maintainer of dbt-core

### Short description

* dbt-core has overlaps in its build dependencies and functional ones (e.g. `packages`)
* setuptools 71+ no longer vends its dependencies and will rely on dependencies installed in the system
* this means the wheel install could change the python dependencies before source distribution install and validation
* we've fixed this by swapping the order in 1.3 and below (since the `packages` conflict only arose there), but a better long term fix would be to build and check wheels and source dists in separate venvs to avoid package conflicts

### Acceptance criteria

build and check wheels and source dists in separate venvs to avoid package conflicts in the build packages step of workflows/main.yml

### Suggested Tests

n/a

### Impact to Other Teams

n/a

### Will backports be required?

yes, back to and including 1.4

### Context

_No response_