environment: fix environment process locator

Fix a bug in the WindowsEnvironment implementation of the
LocateExectuable method. On .NET Core the UseShellExecute property of
ProcessStartInfo defaults to false, whereas on .NET Framework (the
runtime that we target on Windows) defaults to true. You must set this
to false if you want to redirect standard streams (which we want to do).

The .NET Framework-targeting build on Windows was throwing an exception
here(!)

src/shared/Microsoft.Git.CredentialManager/Interop/Posix/PosixEnvironment.cs

@@ -36,6 +36,7 @@ public override string LocateExecutable(string program)
             const string whichPath = "/usr/bin/which";
             var psi = new ProcessStartInfo(whichPath, program)
             {
+                UseShellExecute = false,
                 RedirectStandardOutput = true
             };
 

src/shared/Microsoft.Git.CredentialManager/Interop/Windows/WindowsEnvironment.cs

@@ -70,6 +70,7 @@ public override string LocateExecutable(string program)
             string wherePath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.System), "where.exe");
             var psi = new ProcessStartInfo(wherePath, program)
             {
+                UseShellExecute = false,
                 RedirectStandardOutput = true
             };