chore: modernize EnvironmentVariableService

Replace string.Compare(.., true) with StringComparison.OrdinalIgnoreCase. Short-circuit to Environment.GetEnvironmentVariable on Windows, where it is already case-insensitive. Drop #nullable disable on the file.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: JamieMagee

src/Microsoft.ComponentDetection.Common/EnvironmentVariableService.cs

@@ -1,4 +1,3 @@
-#nullable disable
 namespace Microsoft.ComponentDetection.Common;
 
 using System;
@@ -13,14 +12,19 @@ public bool DoesEnvironmentVariableExist(string name)
         return this.GetEnvironmentVariable(name) != null;
     }
 
-    public string GetEnvironmentVariable(string name)
+    public string? GetEnvironmentVariable(string name)
     {
         // Environment variables are case-insensitive on Windows, and case-sensitive on
         // Linux and MacOS.
         // https://docs.microsoft.com/en-us/dotnet/api/system.environment.getenvironmentvariable
+        if (OperatingSystem.IsWindows())
+        {
+            return Environment.GetEnvironmentVariable(name);
+        }
+
         var caseInsensitiveName = Environment.GetEnvironmentVariables().Keys
             .OfType<string>()
-            .FirstOrDefault(x => string.Compare(x, name, true) == 0);
+            .FirstOrDefault(x => string.Equals(x, name, StringComparison.OrdinalIgnoreCase));
 
         return caseInsensitiveName != null ? Environment.GetEnvironmentVariable(caseInsensitiveName) : null;
     }

src/Microsoft.ComponentDetection.Contracts/IEnvironmentVariableService.cs

@@ -19,7 +19,7 @@ public interface IEnvironmentVariableService
     /// </summary>
     /// <param name="name">Name of the environment variable.</param>
     /// <returns>Returns a string of the environment variable value.</returns>
-    string GetEnvironmentVariable(string name);
+    string? GetEnvironmentVariable(string name);
 
     /// <summary>
     /// Returns the value of an environment variable which is formatted as a delimited list.