Allow for overriding `build_command` #11

Job	Run time
Tests (Ruby 3.1, gemfiles/rails_6_1.gemfile)	1m 40s
Tests (Ruby 3.1, gemfiles/rails_7_0.gemfile)	57s
Tests (Ruby 3.1, gemfiles/rails_7_1.gemfile)	1m 6s
Tests (Ruby 3.1, gemfiles/rails_7_2.gemfile)	1m 6s
Tests (Ruby 3.1, gemfiles/rails_main.gemfile)	1m 27s
Tests (Ruby 3.2, gemfiles/rails_6_1.gemfile)	1m 32s
Tests (Ruby 3.2, gemfiles/rails_7_0.gemfile)	50s
Tests (Ruby 3.2, gemfiles/rails_7_1.gemfile)	1m 2s
Tests (Ruby 3.2, gemfiles/rails_7_2.gemfile)	1m 1s
Tests (Ruby 3.2, gemfiles/rails_main.gemfile)	57s
Tests (Ruby 3.3, gemfiles/rails_6_1.gemfile)	1m 40s
Tests (Ruby 3.3, gemfiles/rails_7_0.gemfile)	1m 0s
Tests (Ruby 3.3, gemfiles/rails_7_1.gemfile)	1m 11s
Tests (Ruby 3.3, gemfiles/rails_7_2.gemfile)	1m 7s
Tests (Ruby 3.3, gemfiles/rails_main.gemfile)	58s
	17m 34s

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