Disable triggers for _collation during bootstrap

After patch that introduced "none" collation (a953051e),
box.internal.bootstrap() started to fail due to inability to drop
mentioned collation. Lets turn off system triggers for _collation space
in order to process its complete purification during bootstrap.

src/box/lua/upgrade.lua

@@ -70,6 +70,7 @@ local function set_system_triggers(val)
     box.space._func:run_triggers(val)
     box.space._priv:run_triggers(val)
     box.space._trigger:run_triggers(val)
+    box.space._collation:run_triggers(val)
 end
 
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