[Cosmos DB] Fixing Cosmos emulator CI

[ealsur]

Emulator CI started to fail on 8/31 with:

"C:\Program Files\PowerShell\7\pwsh.exe" -NoLogo -NoProfile -NonInteractive -ExecutionPolicy Unrestricted -Command ". 'D:\a\_work\_temp\71e34255-01cf-49e5-b060-8a3f2ac8beaf.ps1'"
UseAzcoreFromMain : The term 'UseAzcoreFromMain' is not recognized as a name of a cmdlet, function, script file, or executable program.
Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
At D:\a\_work\_temp\71e34255-01cf-49e5-b060-8a3f2ac8beaf.ps1:4 char:64
+ … k\1\s\eng\scripts\Build_Perf.ps1' data/azcosmos $$(UseAzcoreFromMain)
+                                                      ~~~~~~~~~~~~~~~~~
+ CategoryInfo          : ObjectNotFound: (UseAzcoreFromMain:String) [], ParentContainsErrorRecordException
+ FullyQualifiedErrorId : CommandNotFoundException
##[error]PowerShell exited with code '1'.
Finishing: Build Performance Tests

Apparently this is due to a missing global variable.