Fix process handle leak when launching a job container (#2188)

CreateProcess gives us back a handle to the newly created process.
Previously, we ignored this handle, which meant it was leaking every
time we created a new job container (or anything else that uses
internal/exec in the future).

Process handle leaks can be bad as an exited process is left as a
"zombie" until all handles to it have closed, continuing to use memory.

Fix this by closing the handle from CreateProcess.

Signed-off-by: Kevin Parsons <kevpar@microsoft.com>
Co-authored-by: Kevin Parsons <kevpar@microsoft.com>

internal/exec/exec.go

@@ -209,8 +209,8 @@ func (e *Exec) Start() error {
 	if err != nil {
 		return fmt.Errorf("failed to create process: %w", err)
 	}
-	// Don't need the thread handle for anything.
 	defer func() {
+		_ = windows.CloseHandle(windows.Handle(pi.Process))
 		_ = windows.CloseHandle(windows.Handle(pi.Thread))
 	}()