Streamline expression in lib/arch.rb

Previously this triggered an infinite loop in rubocop, but we just had
the opportunity (given the `standard` gem) to upgrade to the newest
version that fixes it.

I reported that bug: rubocop/rubocop#12307

rhizome/common/lib/arch.rb

@@ -4,15 +4,14 @@
 
 ArchClass = Struct.new(:sym) {
   def self.from_system
-    sym = case RbConfig::CONFIG.fetch("target_cpu").downcase
+    new case RbConfig::CONFIG.fetch("target_cpu").downcase
     when /arm64|aarch64/
       "arm64"
     when /amd64|x86_64|x64/
       "x64"
     else
       fail "BUG: could not detect architecture"
     end.intern
-    new sym
   end
 
   def arm64?