fix(wwctl): Create overlay edit tempfile in tmpdir

When editing an overlay, the temporary file was created within the
overlay directory structure. This could lead to the temporary file
being included in the overlay if not cleaned up properly.

This change modifies the behavior to create the temporary file in the
system's default temporary directory, avoiding any potential issues
with the overlay itself.

Not using this change the temporary file can be seen in the overlay:

 DEBUG  : Using temporary file /usr/share/warewulf/overlays/host/ww-overlay-edit-2879742493
 DEBUG  : Checking if path exists as a file: /usr/share/warewulf/overlays/host/rootfs/etc/hosts.ww
 DEBUG  : ExecInteractive(tee, [/usr/share/warewulf/overlays/host/ww-overlay-edit-2879742493])

 # find /srv | grep ww-overlay
 /srv/warewulf/overlays/host/ww-overlay-edit-2879742493

With this patch applied:

 DEBUG  : Using temporary file /tmp/ww-overlay-edit-266752840
 DEBUG  : Checking if path exists as a file: /usr/share/warewulf/overlays/host/rootfs/etc/hosts.ww
 DEBUG  : ExecInteractive(tee, [/tmp/ww-overlay-edit-266752840])

 # find /srv | grep ww-overlay

Assisted-by: Gemini 2.5 Pro
Signed-off-by: Adrian Reber <areber@redhat.com>

CHANGELOG.md

@@ -20,6 +20,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Refactored overlay class. #1968
 - `wwclient` places files from the runtime overlay atomically. #1307, #1975
 - `wwclient` skips files that do not appear to have been modified. #1984
+- `wwctl overlay edit` again writes temporary files to the default location.
+  (After #1886 better resolves the underlying issue from #1473.) #1946
 
 ### Fixed