fix(ci): use proper way how to build a gem from other directory

[bednar]

Proposed Changes

Fixed nightly build:

• Restore gem build behavior and introduce the "-C" flag to gem build rubygems/rubygems#2596
• Improve gem build -C flag rubygems/rubygems#3983

Checklist

• Rebased/mergeable
• rake test completes successfully
• Commit messages are in semantic format
• Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

Merging #92 (fd3ff2d) into master (671d05f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #92   +/-   ##
=======================================
  Coverage   98.99%   98.99%           
=======================================
  Files          16       16           
  Lines         794      794           
=======================================
  Hits          786      786           
  Misses          8        8           

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