Linuxbrew: Use xdg-open

Closes Homebrew/homebrew#29817.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/os.rb

@@ -6,4 +6,13 @@ module OS
   def self.linux?
     /linux/i === RUBY_PLATFORM
   end
+
+  # Utilities
+  if OS.mac?
+    PATH_OPEN = "/usr/bin/open"
+  elsif OS.linux?
+    PATH_OPEN = "xdg-open"
+  else
+    raise "Unknown operating system"
+  end
 end

Library/Homebrew/utils.rb

@@ -206,7 +206,7 @@ def exec_editor *args
 end
 
 def exec_browser *args
-  browser = ENV['HOMEBREW_BROWSER'] || ENV['BROWSER'] || "open"
+  browser = ENV['HOMEBREW_BROWSER'] || ENV['BROWSER'] || OS::PATH_OPEN
   safe_exec(browser, *args)
 end