fix: remove on push to main from matrix workflow

[sbillinge]

Problem

skpkg is still building the matrix workflow with a push to main.

Proposed solution

delete the on push to main block so it can still be triggered by workflow dispatch but doesn't go on merge to main.

[ycexiao]

@sbillinge Could you please check what this issue is about? I only find the workflow matrix-and-codecov-on-merge-to-main.yml relevant in this repo, and it has workflow_dispatch so it should be triggered by workflow dispatch.

name: CI

on:
  push:
    branches:
      - main
  release:
    types:
      - prereleased
      - published
  workflow_dispatch:

[sbillinge]

@ycexiao I think we need to comment out the push | branches | main block, but please check other diffpy packages for consistency.

If we merge this we don't need to push out a new release as it is only used by devs.