Refactor tools.nim to use newly implemented processID() function

$nim_D/nimble/src/nimblepkg/tools.nim:
```nim
proc getNimbleTempDir*(): string =
  ## Returns a path to a temporary directory.
  ##
  ## The returned path will be the same for the duration of the process but
  ## different for different runs of it. You have to make sure to create it
  ## first. In release builds the directory will be removed when nimble finishes
  ## its work.
  result = getTempDir() / "nimble_"
  when defined(windows):
    proc GetCurrentProcessId(): int32 {.stdcall, dynlib: "kernel32",
                                        importc: "GetCurrentProcessId".}
    result.add($GetCurrentProcessId())
  else:
    result.add($getpid())
```

pending https://github.com/nim-lang/Nim/pull/9846
should reuse `osproc.processID() `

also, as mentioned by @data-man here https://github.com/nim-lang/Nim/pull/9846#discussion_r238429042, it should be uint32, not int32 