Fix `make source_archive_complete` for release branches

In Blender 3.1 we can't run the source_archive_complete because the
cmake program is trying to download the packages from svn trunk. However
3.2 (aka master) already changed the version of some of the source
packages.

For example the OpenXR-SDK. It should be looking for
OpenXR-SDK-1.0.17.tar.gz in:
https://svn.blender.org/svnroot/bf-blender/tags/blender-3.1-release/lib/packages/

But instead it tries to look for it in:
https://svn.blender.org/svnroot/bf-blender/trunk/lib/packages/

Which can't be found since it was replaced with OpenXR-SDK-1.0.22.tar.gz

---

Release checklist: https://wiki.blender.org/wiki/Process/Release_Checklist

The release checklist was updated to include the new instructions:
`In the release branch, update and uncomment BLENDER_VERSION in download.cmake`

Differential Revision: http://developer.blender.org/D14292

build_files/build_environment/cmake/download.cmake

@@ -1,11 +1,16 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 
+## Update and uncomment this in the release branch
+# set(BLENDER_VERSION 3.1)
+
 function(download_source dep)
   set(TARGET_FILE ${${dep}_FILE})
   set(TARGET_HASH_TYPE ${${dep}_HASH_TYPE})
   set(TARGET_HASH ${${dep}_HASH})
   if(PACKAGE_USE_UPSTREAM_SOURCES)
     set(TARGET_URI  ${${dep}_URI})
+  elseif(BLENDER_VERSION)
+    set(TARGET_URI  https://svn.blender.org/svnroot/bf-blender/tags/blender-${BLENDER_VERSION}-release/lib/packages/${TARGET_FILE})
   else()
     set(TARGET_URI  https://svn.blender.org/svnroot/bf-blender/trunk/lib/packages/${TARGET_FILE})
   endif()