CI: bust corrupted Yarn fallback build cache

The web compile step failed packing the git-sourced @replit/codemirror-*
deps with 'tar content corrupt' / EEXIST / missing .yarn-tarball.tgz errors,
all under /usr/local/share/.cache/yarn/v6 — i.e. a corrupted BuildKit
fallback-cache mount (likely left half-written by an interrupted build), not
a code or dependency change. Bump the fallback cache id so BuildKit
allocates a fresh empty cache; the berry and webpack caches are untouched.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server-ce/Dockerfile

@@ -27,7 +27,7 @@ ENV COREPACK_ENABLE_NETWORK=0
 # -------------------------
 RUN --mount=type=cache,target=/root/.cache \
     --mount=type=cache,target=/root/.yarn/berry/cache,id=server-ce-yarn-cache \
-    --mount=type=cache,target=/usr/local/share/.cache/yarn,id=server-ce-yarn-fallback-cache \
+    --mount=type=cache,target=/usr/local/share/.cache/yarn,id=server-ce-yarn-fallback-cache-v2 \
     --mount=type=tmpfs,target=/tmp node genScript install | bash
 
 # Add the actual source files
@@ -35,7 +35,7 @@ RUN --mount=type=cache,target=/root/.cache \
 COPY --parents libraries/ services/ tools/migrations/ /overleaf/
 RUN --mount=type=cache,target=/root/.cache \
     --mount=type=cache,target=/root/.yarn/berry/cache,id=server-ce-yarn-cache \
-    --mount=type=cache,target=/usr/local/share/.cache/yarn,id=server-ce-yarn-fallback-cache \
+    --mount=type=cache,target=/usr/local/share/.cache/yarn,id=server-ce-yarn-fallback-cache-v2 \
     --mount=type=cache,target=/overleaf/services/web/node_modules/.cache,id=server-ce-webpack-cache \
     --mount=type=tmpfs,target=/tmp \
     node genScript compile | bash || \