fix(whitby): Move Restic's cache into /var/backup/restic

It tries to write this to ~/.cache otherwise, which worked for the git
user but does not work for root (??)

Change-Id: I02d04da7d8e2b8782ce70bc72bce0b90c3961aa0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1546
Reviewed-by: glittershark <grfn@gws.fyi>
Tested-by: BuildkiteCI

ops/nixos/whitby/default.nix

@@ -296,6 +296,7 @@ in lib.fix(self: {
       GOOGLE_APPLICATION_CREDENTIALS = "/var/backup/restic/gcp-key.json";
       RESTIC_REPOSITORY = "gs:tvl-fyi-backups:/whitby";
       RESTIC_PASSWORD_FILE = "/var/backup/restic/secret";
+      RESTIC_CACHE_DIR = "/var/backup/restic/cache";
       RESTIC_EXCLUDE_FILE = builtins.toFile "exclude-files" ''
         /var/lib/gerrit/tmp
       '';