set approriate coverage key, use it as command

.gitlab-ci.yml

@@ -111,7 +111,7 @@ regression tests:
   variables:
     BUNDLE_WITHOUT: lint:assorted
     CI: 1
-    COVERAGE_KEY: "$RUBY_ENGINE-$RUBY_VERSION-regression-tests"
+    COVERAGE_KEY: "ruby-3.4-regression-tests"
   artifacts:
     paths:
       - coverage/

test/test_helper.rb

@@ -4,8 +4,8 @@ GC.auto_compact = true if !defined?(MimeMagic) && GC.respond_to?(:auto_compact=)
 
 if ENV.key?("CI")
   require "simplecov"
-  SimpleCov.command_name "#{RUBY_ENGINE}-#{RUBY_VERSION}"
   coverage_key = ENV.fetch("COVERAGE_KEY", "#{RUBY_ENGINE}-#{RUBY_VERSION}")
+  SimpleCov.command_name coverage_key
   SimpleCov.coverage_dir "coverage/#{coverage_key}"
 end