Extract a gem_command method

And use it consistently.

lib/bundler/gem_helper.rb

@@ -73,8 +73,7 @@ def install
 
     def build_gem
       file_name = nil
-      gem = ENV["GEM_COMMAND"] ? ENV["GEM_COMMAND"] : "gem"
-      sh("#{gem} build -V #{spec_path}".shellsplit) do
+      sh("#{gem_command} build -V #{spec_path}".shellsplit) do
         file_name = File.basename(built_gem_path)
         SharedHelpers.filesystem_access(File.join(base, "pkg")) {|p| FileUtils.mkdir_p(p) }
         FileUtils.mv(built_gem_path, "pkg")
@@ -85,8 +84,7 @@ def build_gem
 
     def install_gem(built_gem_path = nil, local = false)
       built_gem_path ||= build_gem
-      gem = ENV["GEM_COMMAND"] ? ENV["GEM_COMMAND"] : "gem"
-      cmd = "#{gem} install #{built_gem_path}"
+      cmd = "#{gem_command} install #{built_gem_path}"
       cmd += " --local" if local
       out, status = sh_with_status(cmd.shellsplit)
       unless status.success? && out[/Successfully installed/]
@@ -98,7 +96,7 @@ def install_gem(built_gem_path = nil, local = false)
   protected
 
     def rubygem_push(path)
-      cmd = %W[gem push #{path}]
+      cmd = %W[#{gem_command} push #{path}]
       cmd << "--key" << gem_key if gem_key
       cmd << "--host" << allowed_push_host if allowed_push_host
       unless allowed_push_host || Bundler.user_home.join(".gem/credentials").file?
@@ -211,5 +209,9 @@ def gem_key
     def gem_push?
       !%w[n no nil false off 0].include?(ENV["gem_push"].to_s.downcase)
     end
+
+    def gem_command
+      ENV["GEM_COMMAND"] ? ENV["GEM_COMMAND"] : "gem"
+    end
   end
 end