FIX: Estimate free memory with "available", not "free" (#2985)

## Changes proposed in this pull request

According to the [psutil
docs](https://psutil.readthedocs.io/en/latest/#psutil.virtual_memory):

> Return statistics about system memory usage as a named tuple including
the following fields, expressed in bytes. Main metrics:
> 
> * total: total physical memory (exclusive swap).
> * available: the memory that can be given instantly to processes
without the system going into swap. This is calculated by summing
different memory values depending on the platform and it is supposed to
be used to monitor actual memory usage in a cross platform fashion.
> 
> Other metrics:
> 
> * used: memory used, calculated differently depending on the platform
and designed for informational purposes only. total - free does not
necessarily match used.
> * free: memory not being used at all (zeroed) that is readily
available; note that this doesn’t reflect the actual memory available
(use available instead). total - used does not necessarily match free.

Running locally, `virtual_memory().available` matches what I see in
`top`, not `virtual_memory().free`.

This was found while debugging an issue with `unwarp_wf.resample`, which
selects `num_threads` based on the memory available.

fmriprep/config.py

@@ -178,7 +178,7 @@
 try:
     from psutil import virtual_memory
 
-    _free_mem_at_start = round(virtual_memory().free / 1024**3, 1)
+    _free_mem_at_start = round(virtual_memory().available / 1024**3, 1)
 except Exception:
     _free_mem_at_start = None