Set osOverride default value to empty on mono builds

[safern]

I was looking at one mono build and noticed that for legs that don't set this value, a $(osOverride) literal is passed down to the scripts. Builds don't fail, but logs look weird and there is also a error in the script:
mono.cmd -configuration debug -arch x64 $(osOverride) -ci /p:MonoEnableLLVM=false

osOverride : The term 'osOverride' is not recognized as the name of a cmdlet, function, script file, or operable 
program. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
At line:1 char:92
+ ... bsetCategory mono -configuration debug -arch x64 $(osOverride) -ci /p ...
+                                                        ~~~~~~~~~~
    + CategoryInfo          : ObjectNotFound: (osOverride:String) [], CommandNotFoundException
    + FullyQualifiedErrorId : CommandNotFoundException

[safern]

Failure is: #33928