Fake.Core.Process: stop using old ref-cell operators

The new version of the F# compiler now flags these occurrences as
deprecated.

The messages are of two kinds:

1) info FS3370: The use of ':=' from the F# library is deprecated.
See https://aka.ms/fsharp-refcell-ops.
For example, please change 'cell := expr' to 'cell.Value <- expr'.

2) info FS3370: The use of '!' from the F# library is deprecated.
See https://aka.ms/fsharp-refcell-ops.
For example, please change '!cell' to 'cell.Value'.

src/app/Fake.Core.Process/InternalStreams.fs

@@ -100,13 +100,13 @@ module internal InternalStreams =
  let asyncResult = ref null
  Async.FromBeginEnd(
  (fun (callback, state) -> 
- asyncResult := beginAction(callback, state)
- !asyncResult), 
+ asyncResult.Value <- beginAction(callback, state)
+ asyncResult.Value),
  (fun res -> 
  endAction res), 
  cancelAction = (fun () -> 
- while !asyncResult = null do Thread.Sleep 20
- cancelAction(!asyncResult)))
+ while asyncResult.Value = null do Thread.Sleep 20
+ cancelAction(asyncResult.Value)))
  open AsyncHelper
 
  type ConcurrentQueueMessage<'a> =
@@ -232,12 +232,12 @@ module internal InternalStreams =
  while not cts.IsCancellationRequested do
  let! data = innerStream.Read()
  let finished = ref false
- while not !finished do
+ while not finished.Value do
  let! (asyncResult:MyIAsyncReadResult<'a>) = queue.DequeAsync()
  if not asyncResult.IsCanceled then
  try
  asyncResult.End(data)
- finished := true
+ finished.Value <- true
  with ReadCanceledException -> () // find next
  return ()
  }, cancellationToken = workerCts.Token)
@@ -466,7 +466,7 @@ module internal InternalStreams =
  //let raw = infiniteStream()
  let readFinished = ref false
  let read () =
- if !readFinished then
+ if readFinished.Value then
  async.Return None
  else 
  async {
@@ -477,16 +477,16 @@ module internal InternalStreams =
  match s with
  | Some d -> Some d
  | None -> 
- readFinished := true
+ readFinished.Value <- true
  None
  | None -> 
  failwith "stream should not be limited as we are using an infiniteStream!" }
  let isFinished = ref false
  let finish () = async {
  do! raw.Write None
- isFinished := true }
+ isFinished.Value <- true }
  let write item = 
- if !isFinished then
+ if isFinished.Value then
  failwith "stream is in finished state so it should not be written to!"
  raw.Write (Some item)
  finish,