Added OS defaults for Python executable (#4934)

### Description
Fixes #4922 
Adds default python executable name by detected OS.

### How has this been tested?
Not tested

<small>By submitting this pull request, I confirm that my contribution
is made under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).</small>

Source/DafnyCore/Compilers/Python/PythonBackend.cs

@@ -4,6 +4,7 @@
 using System.IO;
 using System.Linq;
 using System.Text.RegularExpressions;
+using System.Runtime.InteropServices;
 
 namespace Microsoft.Dafny.Compilers;
 
@@ -46,6 +47,17 @@ private static string FindModuleName(string externFilename) {
     return Path.GetExtension(externFilename) == ".py" ? Path.GetFileNameWithoutExtension(externFilename) : null;
   }
 
+  private static readonly Dictionary<OSPlatform, string> PlatformDefaults = new() {
+    {OSPlatform.Linux, "python3"},
+    {OSPlatform.Windows, "python"},
+    {OSPlatform.FreeBSD, "python3"},
+    {OSPlatform.OSX, "python3"},
+  };
+  private static string DefaultPythonCommand => PlatformDefaults.SingleOrDefault(
+      kv => RuntimeInformation.IsOSPlatform(kv.Key),
+      new(OSPlatform.Linux, "python3")
+    ).Value;
+
   bool CopyExternLibraryIntoPlace(string externFilename, string mainProgram, TextWriter outputWriter) {
     // Grossly, we need to look in the file to figure out where to put it
     var moduleName = FindModuleName(externFilename);
@@ -88,7 +100,7 @@ public override bool CompileTargetProgram(string dafnyProgramName, string target
       }
     }
     if (!runAfterCompile) {
-      var psi = PrepareProcessStartInfo("python3");
+      var psi = PrepareProcessStartInfo(DefaultPythonCommand);
       psi.Arguments = $"-m compileall -q {Path.GetDirectoryName(targetFilename)}";
       return 0 == RunProcess(psi, outputWriter, outputWriter, "Error while compiling Python files.");
     }
@@ -99,7 +111,7 @@ public override bool RunTargetProgram(string dafnyProgramName, string targetProg
     string targetFilename, ReadOnlyCollection<string> otherFileNames, object compilationResult, TextWriter outputWriter,
     TextWriter errorWriter) {
     Contract.Requires(targetFilename != null || otherFileNames.Count == 0);
-    var psi = PrepareProcessStartInfo("python3", Options.MainArgs.Prepend(targetFilename));
+    var psi = PrepareProcessStartInfo(DefaultPythonCommand, Options.MainArgs.Prepend(targetFilename));
     psi.EnvironmentVariables["PYTHONIOENCODING"] = "utf8";
     return 0 == RunProcess(psi, outputWriter, errorWriter);
   }