Move nix-cache's binary cache signing key into sops

nix-serve's secretKeyFile was a manual, undocumented-outside-a-comment
`nix-store --generate-binary-cache-key` step per host -- easy to miss on
a fresh nix-cache instance (as lxc-nix-cache testing just found: systemd
fails the unit with EXIT_CREDENTIALS when LoadCredential can't find the
source file, which nginx then reports as a 502 from clients). It also
can't be regenerated per-host safely: modules/nix-cache/client.nix
hardcodes every client's trust in one specific public key, so every
nix-cache instance has to share the exact same keypair.

Sourced from secrets/nix-cache.yaml's new cache-priv-key entry instead,
via the same sops-nix pattern every other secret in this repo already
uses. Verified the added value derives to the exact public key
modules/nix-cache/client.nix already trusts before committing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot

secrets/nix-cache.yaml

@@ -1,4 +1,5 @@
 beszel-token: ENC[AES256_GCM,data:meuzUP/6wCssJDVTgbC0XwiLZPMGyDl55HEIiON9xOXCD9k6,iv:TDqWcp+8Mxd8wN09r5otQRQXq3XTeQphaTWxvvuLTAs=,tag:cRPZQGlwB/dTguBAheWPQg==,type:str]
+cache-priv-key: ENC[AES256_GCM,data:6vQKIf7eS0WNL2Eptoi4VWr18SRMZfN/H/aFUUtXdMYQY5LLyBp2EHRKqZcGFuh1nZhUdAxUztq/CVXx+QFxKW+ElHxCxUSp0QqI1fdSkBkKZb8hlit5SoX9JtLzZGg0HBNM3nJu,iv:0J+xmrPJhInHhFR/c41ACjuTfaIoMkQFSfbL2KkgFa8=,tag:f4s9Szs5oprVVRSyXaX48A==,type:str]
 sops:
     age:
         - enc: |
@@ -28,7 +29,7 @@ sops:
             2SEMsctdGC+5E3ilPXvPpZ5RONZHbXxn6kQRBlBv6AJERpGDzsfgfA==
             -----END AGE ENCRYPTED FILE-----
           recipient: age164px2a8e48ptsf9ngtan38aa6jls4jdl26mzrgzf6sn3vcvt49hqjrgr8w
-    lastmodified: "2026-07-19T02:30:40Z"
-    mac: ENC[AES256_GCM,data:7+FeT6aeCGn+JFBXbPO0qP4BJ1nHPSennewv1kWkG+hOTIqs1ymuswUK1Hyfi6Z9h2umFX9HvK+o3qtmYvk6k7BUNe6w6QUHTNwm6lmNqrb7sgAE3iFVI2p9m14NGhgoTfnXx1M4JIZ1iuNYhCukpENI4+svIe+r7x5YeE5Evac=,iv:4AKAPI6upyAvHBr8BLWX7R/NupmJdcXdqiN8e0ZQ3ls=,tag:TkpfpCaLhn+Mx5cQZZuMdA==,type:str]
+    lastmodified: "2026-07-19T23:30:21Z"
+    mac: ENC[AES256_GCM,data:kLGE2xawQT7mx+sfw68hmGk5nCEGiEjZrqTEl9B1dtQmTrMwmoVr/1RISi4LfJrwxy31mDgff4lcIL4wIJuM373uk3X8j4RNyYQNTfKEkORT6r8NHeepNs267O77pKGd7OmcM4MT/BqOnB8ELS7Wlf2ect7CAlvUUVyc8icxgZE=,iv:EYLDsHYHZ1XOQXafOTqHHWpk/OBNq/R6IJnOBYV33E4=,tag:thxrCPC5oGvDjhK7Dz87YA==,type:str]
     unencrypted_suffix: _unencrypted
     version: 3.13.1