fix(ci): create the profile dir so macOS nix prune keeps the toolchain warm

The prune step's profile path $HOME/.cache/rdbms-ci/toolchain had no
parent dir, so `nix develop --profile` errored ("cannot read directory")
and — swallowed by `|| true` — never created the profile/gc-root. With
nothing rooting the toolchain, nix-collect-garbage deleted the whole
closure every run (~3.8 GiB re-downloaded each dispatch; confirmed in the
run-74 log). Add `mkdir -p` for the parent and drop the `|| true` on the
profile realization so a future breakage fails loudly. The VM stays
bounded as before, but the toolchain now persists across runs.

release-macos.yaml is workflow_dispatch (runs from main's definition), so
this takes effect on the next dispatch — the already-published v0.2.0
macOS binaries are unaffected.

.gitea/workflows/release-macos.yaml

@@ -84,12 +84,21 @@ jobs:
         # The runner wipes the workspace each run, so cargo target/ never
         # accumulates. Bound the persistent nix store by generation: record the
         # current devShell as a generation of a persistent profile (in $HOME),
-        # keep the 2 newest, reclaim what older ones referenced.
+        # keep the 2 newest, reclaim what older ones referenced — so the
+        # toolchain stays *warm* across runs and only stale generations are GC'd.
+        #
+        # The profile's parent dir MUST exist first, or `nix develop --profile`
+        # errors ("cannot read directory …") and the profile/gc-root is never
+        # created — which made `nix-collect-garbage` delete the whole toolchain
+        # closure every run (re-downloaded ~3.8 GiB each time; the retention was
+        # silently broken by the swallowed `|| true`). No `|| true` on the
+        # profile realization now: a future breakage should fail loudly.
         if: always()
         run: |
           echo "--- disk before ---"; df -h / | tail -1
           P="$HOME/.cache/rdbms-ci/toolchain"
-          nix develop --profile "$P" -c true || true
+          mkdir -p "$(dirname "$P")"
+          nix develop --profile "$P" -c true
           nix-env -p "$P" --delete-generations +2 || true
           nix-collect-garbage || true
           echo "--- disk after ---"; df -h / | tail -1