Fix DependencyManager bug (#15391)

Author: 0101

src/FSharp.DependencyManager.Nuget/FSharp.DependencyManager.Utilities.fs

@@ -197,7 +197,7 @@ module internal Utilities =
             psi.EnvironmentVariables.Remove("MSBuildSDKsPath") // Host can sometimes add this, and it can break things
 
             for varname, value in environment do
-                psi.EnvironmentVariables.Add(varname, value)
+                psi.EnvironmentVariables[ varname ] <- value
 
             psi.UseShellExecute <- false