chore: improve CI workflow stability

[romanzac]

I would like to receive team's feedback on several changes I've prepared to address CI workflow stability.

This PR introduces possibility to preset Nimble version during Nim 1.x build. Benefits of such change are:

• Possible to have any version of Nimble even for Nim v1.x.
• Get native Nimble lock functionality for Nim v1.x.
• Keep Nimble versions same across different Nim versions -> simplified troubleshooting
• Get Nimble issues fixed faster - the newest versions usually get preference

Caching is disabled for CI workflow temporarily. Once we have more stability, we can start tuning for speed.

[codecov]

Codecov Report

Merging #1004 (c406613) into unstable (3011ba4) will decrease coverage by 0.04%.
The diff coverage is n/a.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1004      +/-   ##
============================================
- Coverage     83.16%   83.12%   -0.04%     
============================================
  Files            91       91              
  Lines         15327    15327              
============================================
- Hits          12746    12741       -5     
- Misses         2581     2586       +5     

see 3 files with indirect coverage changes