Added changes entry

Signed-off-by: Ambre Austen Suhamy <[email protected]>

Author: ElectreAAS

doc/changes/fixed/11612.md

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+- Fixed non-build caches not following `$DUNE_CACHE_ROOT` and instead only
+  relying on `$XDG_CACHE_HOME`.
+  This means the normal build cache moves:
+  `$DUNE_CACHE_ROOT -> $DUNE_CACHE_ROOT/db` (no changes if that variable was
+  unset). Affected users can prevent a full cache invalidation by moving
+  previous contents:
+  `cd $DUNE_CACHE_ROOT; mkdir db; mv <contents of directory> db`.
+  (#11612, fixes #11584, @ElectreAAS)