refactor: Minor improvements to IsLinuxGoogleBiosAsync

- Avoid having the same string literal twice
- Use the simpler File.OpenText instead of constructing a StreamReader explicitly
- Use a `using` declaration to simplify the rest of the code

Fixes b/335638570

Author: jskeet

Src/Support/Google.Apis.Auth/OAuth2/ComputeCredential.cs

@@ -477,11 +477,8 @@ async Task<bool> IsLinuxGoogleBiosAsync()
                     return false;
                 }
 
-                string productName;
-                using (var streamReader = new StreamReader(new FileStream("/sys/class/dmi/id/product_name", FileMode.Open, FileAccess.Read)))
-                {
-                    productName = await streamReader.ReadLineAsync().ConfigureAwait(false);
-                }
+                using var reader = File.OpenText(fileName);
+                string productName = await reader.ReadLineAsync().ConfigureAwait(false);
                 productName = productName?.Trim();
                 return productName == "Google" || productName == "Google Compute Engine";
             }