Use non-subshell `system` variant in installer #431

Job	Run time
Tests (Ruby 2.7, Gemfile)	2m 51s
Tests (Ruby 2.7, gemfiles/rails_7_0_propshaft.gemfile)	1m 1s
Tests (Ruby 2.7, gemfiles/rails_7_1_propshaft.gemfile)	1m 3s
Tests (Ruby 2.7, gemfiles/rails_7_0_sprockets.gemfile)	57s
Tests (Ruby 2.7, gemfiles/rails_7_1_sprockets.gemfile)	1m 9s
Tests (Ruby 3.0, Gemfile)	1m 39s
Tests (Ruby 3.0, gemfiles/rails_7_0_propshaft.gemfile)	39s
Tests (Ruby 3.0, gemfiles/rails_7_1_propshaft.gemfile)	43s
Tests (Ruby 3.0, gemfiles/rails_7_0_sprockets.gemfile)	42s
Tests (Ruby 3.0, gemfiles/rails_7_1_sprockets.gemfile)	52s
Tests (Ruby 3.1, Gemfile)	1m 36s
Tests (Ruby 3.1, gemfiles/rails_7_0_propshaft.gemfile)	41s
Tests (Ruby 3.1, gemfiles/rails_7_1_propshaft.gemfile)	52s
Tests (Ruby 3.1, gemfiles/rails_7_0_sprockets.gemfile)	40s
Tests (Ruby 3.1, gemfiles/rails_7_1_sprockets.gemfile)	44s
Tests (Ruby 3.2, Gemfile)	1m 37s
Tests (Ruby 3.2, gemfiles/rails_7_0_propshaft.gemfile)	46s
Tests (Ruby 3.2, gemfiles/rails_7_1_propshaft.gemfile)	49s
Tests (Ruby 3.2, gemfiles/rails_7_0_sprockets.gemfile)	43s
Tests (Ruby 3.2, gemfiles/rails_7_1_sprockets.gemfile)	43s
	20m 47s

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