fix(aquamarine): persist the user's cache directory (#466)

On a tmpfs root the flake/fetch cache is empty at every boot, so
`nix run <flake-ref>` re-downloads its input and fails until DNS is up: a reboot
test showed llama-swap's first start failing on that. Other caches write into
RAM meanwhile.

Assisted-by: DeepSeek Harness (deepseek-flash)

outputs/x86_64-linux/tests/idols-aquamarine/expected.nix

@@ -8,6 +8,7 @@
   cStatesCapped = true;
   nativeIpv6Preference = true;
   sourceCheckoutsPersistent = true;
+  cachePersistent = true;
   userUnitDirsPersistent = true;
   rootlessPodmanEnabled = true;
   stableKernel = true;

outputs/x86_64-linux/tests/idols-aquamarine/expr.nix

@@ -37,6 +37,11 @@ in
     d: d.directory == "/home/${myvars.username}/src"
   ) cfg.preservation.preserveAt."/persistent".users.${myvars.username}.directories;
 
+  # A cold flake cache re-downloads the input of a `nix run <flake-ref>` at boot.
+  cachePersistent = builtins.any (
+    d: d.directory == "/home/${myvars.username}/.cache"
+  ) cfg.preservation.preserveAt."/persistent".users.${myvars.username}.directories;
+
   # Directories the tmpfs root would otherwise drop.
   userUnitDirsPersistent =
     let