Fix CI: use x86_64-linux-gnu platform (not x86_64-linux)

The sqlite3 gem 2.x uses x86_64-linux-gnu, not x86_64-linux.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: andreibondarev

Gemfile.lock

@@ -342,7 +342,6 @@ PLATFORMS
   arm64-darwin
   ruby
   x86_64-darwin
-  x86_64-linux
   x86_64-linux-gnu
 
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