just mutants*: scratch copies go to the user's cache, never /tmp

cargo-mutants copies and builds the tree per job in TMPDIR; on Arch /tmp
is tmpfs (RAM, up to half of it), and a GUI run put 20 GB there. Every
recipe now uses $XDG_CACHE_HOME/wanderbooks/mutants (~/.cache/...).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX

.github/workflows/scheduled.yml

@@ -33,7 +33,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: jdx/mise-action@v2
       - run: just mutants
-      - run: TMPDIR=/dev/shm just mutants-store
+      - run: just mutants-store
       - if: always()
         uses: actions/upload-artifact@v4
         with: