don't start the cosmos emulator on Linux builds (#4205)

We made a similar change in `release/azure_data_cosmos-previews`, but it
needs to be included here to avoid launching the emulator on Linux
builds. We were already marking them as skipped, but still
starting/stopping the emulator. This change causes us to skip the entire
emulator start process.

Co-authored-by: Daniel Jurek <djurek@microsoft.com>

Author: analogrelay

sdk/cosmos/azure_data_cosmos/Test-Setup.ps1

@@ -29,6 +29,8 @@ if($IsAzDo) {
         $env:AZURE_COSMOS_TEST_MODE = "required"
     } else {
         $env:AZURE_COSMOS_TEST_MODE = "skipped"
+        Write-Host "Skipping Cosmos DB Emulator setup on non-Windows Azure DevOps agents."
+        return
     }
 }