Buildbot: Switch to devtoolset-9

Famous "should just work", but will never know until actually attempted
 to be used.

Ref T76783.

build_files/buildbot/buildbot_utils.py

@@ -51,7 +51,7 @@ class Builder:
         elif name.startswith('linux'):
             self.platform = 'linux'
             if is_tool('scl'):
-                self.command_prefix =  ['scl', 'enable', 'devtoolset-6', '--']
+                self.command_prefix =  ['scl', 'enable', 'devtoolset-9', '--']
             else:
                 self.command_prefix =  []
         elif name.startswith('win'):