ci: Print cargo target directory sizes

[aborgna-q]

Adds some debug information to track cargo's target directory size in some steps that were running out of disk space.

It seems that just disabling the cache reduces the test-nightly step from 12Gb to 9Gb, so I may do a cache purge.

Some sources recommend disabling incremental builds to reduce space, but that actually caused out-of-space errors in the normal jobs (like test-stable-*) that currently take 7Gb...

drive-by: Split and organize the test-nightly job, to better track the panics if they appear again.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.71%. Comparing base (af20636) to head (7d11ed9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2814      +/-   ##
==========================================
- Coverage   83.71%   83.71%   -0.01%     
==========================================
  Files         261      261              
  Lines       52637    52631       -6     
  Branches    47378    47372       -6     
==========================================
- Hits        44064    44058       -6     
  Misses       6183     6183              
  Partials     2390     2390              

Flag	Coverage Δ
python	88.85% <ø> (ø)
rust	83.13% <ø> (-0.01%)	⬇️

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