ci: fix release notes generation in release workflow

prepare job failed: git-cliff --latest crashed with 'trim_start_matches on
null' because the checkout was shallow (no tags/history) so no release existed.

- checkout prepare with fetch-depth: 0 + fetch-tags so git-cliff sees full
  history and tags (also fixes an otherwise-truncated CHANGELOG)
- generate release notes with --unreleased --tag v$VERSION instead of --latest:
  --latest selected the previous real tag (stale notes); --unreleased --tag
  renders the version being released

dist/chocolatey/termscp.nuspec

@@ -53,6 +53,6 @@ Features:
     </description>
   </metadata>
   <files>
-    <file src="tools\**" target="tools" />
+    <file src="tools/**" target="tools" />
   </files>
 </package>