os: add uname method

This will allow us to replace code like

    os = if OS.mac?
      "Darwin"
    else
      "Linux"
    end

with a call to `OS.uname`. Doing

    rg '= (if )?OS\.(mac|linux)\?'

returns about 70 matches, so there are probably at least a handful of
formulae that could be simplified by this.

Library/Homebrew/os.rb

@@ -35,6 +35,14 @@ module OS
     @kernel_version ||= Version.new(Utils.safe_popen_read("uname", "-r").chomp)
   end
 
+  # Get the kernel name.
+  #
+  # @api public
+  sig { returns(String) }
+  def self.uname
+    @uname ||= Utils.safe_popen_read("uname").chomp
+  end
+
   ::OS_VERSION = ENV["HOMEBREW_OS_VERSION"]
 
   CI_GLIBC_VERSION = "2.23"