Build node and farmer separately to use less RAM in CI

[nazar-pc]

In test run the process was killed, I guessed it is because our codebase gets heavier over time, so compiling took more RAM. Separating farmer and node into separate steps helped CI to pass successfully.

Code contributor checklist:

• I have reviewed my own changes one more time to spot typos, unintended changes, following project conventions, etc.
• I have prepared clean readable history of commits before submitting this PR to make reviewer's life easier
• I have tested my changes and/or added corresponding test cases (if relevant)
• I understand that any changes to this PR going forward will notify multiple developers and will try to minimize them
• I have added sufficient description of changes to make review process easier
• This PR is ready for review by developers

[nazar-pc]

Hm... another job got killed here: https://github.com/subspace/subspace/runs/7490192102?check_suite_focus=true

Not sure why, test run in my repo worked just fine 😕