bottle_arch: ensure a symbol is used when needed.

Fixes #7597.

Library/Homebrew/extend/ENV/shared.rb

@@ -255,7 +255,7 @@ module SharedEnvExtension
   # @private
   def effective_arch
     if Homebrew.args.build_bottle? && Homebrew.args.bottle_arch
-      Homebrew.args.bottle_arch
+      Homebrew.args.bottle_arch.to_sym
     else
       Hardware.oldest_cpu
     end

Library/Homebrew/extend/os/linux/extend/ENV/shared.rb

@@ -3,8 +3,10 @@
 module SharedEnvExtension
   # @private
   def effective_arch
-    if Homebrew.args.build_bottle?
-      Homebrew.args.bottle_arch || Hardware.oldest_cpu
+    if Homebrew.args.build_bottle? && Homebrew.args.bottle_arch
+      Homebrew.args.bottle_arch.to_sym
+    elsif Homebrew.args.build_bottle?
+      Hardware.oldest_cpu
     else
       :native
     end

Library/Homebrew/formula_installer.rb

@@ -269,7 +269,7 @@ class FormulaInstaller
 
     return if only_deps?
 
-    if build_bottle? && (arch = Homebrew.args.bottle_arch) && !Hardware::CPU.optimization_flags.include?(arch)
+    if build_bottle? && (arch = Homebrew.args.bottle_arch) && !Hardware::CPU.optimization_flags.include?(arch.to_sym)
       raise "Unrecognized architecture for --bottle-arch: #{arch}"
     end