Switch to File.OpenRead (#6393)

Context
The precomputed cache from dotnet/installer#10037 lives in Program Files after it's installed on a new computer. Program Files can only be accessed with admin privileges, which not all users have and those that have generally wouldn't expect. This permits reading the precomputed cache even without admin rights.

Changes Made
new FileStream(stateFile, FileMode.Open) opens the file as if you had read/write access but only actually grants you read permissions. It still requires administrator privileges, however, if a file requires administrator privileges to write to. This removes that requirement.

src/Tasks/StateFileBase.cs

@@ -73,7 +73,7 @@ internal static StateFileBase DeserializeCache(string stateFile, TaskLoggingHelp
             {
                 if (!string.IsNullOrEmpty(stateFile) && FileSystems.Default.FileExists(stateFile))
                 {
-                    using (FileStream s = new FileStream(stateFile, FileMode.Open))
+                    using (FileStream s = File.OpenRead(stateFile))
                     {
                         var formatter = new BinaryFormatter();
                         object deserializedObject = formatter.Deserialize(s);

src/Tasks/SystemState.cs

@@ -284,7 +284,7 @@ internal static SystemState DeserializeCacheByTranslator(string stateFile, TaskL
             {
                 if (!string.IsNullOrEmpty(stateFile) && FileSystems.Default.FileExists(stateFile))
                 {
-                    using FileStream s = new FileStream(stateFile, FileMode.Open);
+                    using FileStream s = File.OpenRead(stateFile);
                     var translator = BinaryTranslator.GetReadTranslator(s, buffer:null); // TODO: shared buffering?
 
                     // verify file signature