Set up agenix for secrets management

npins/sources.json

@@ -1,5 +1,20 @@
 {
   "pins": {
+    "agenix": {
+      "type": "GitRelease",
+      "repository": {
+        "type": "GitHub",
+        "owner": "ryantm",
+        "repo": "agenix"
+      },
+      "pre_releases": false,
+      "version_upper_bound": null,
+      "release_prefix": null,
+      "version": "0.15.0",
+      "revision": "564595d0ad4be7277e07fa63b5a991b3c645655d",
+      "url": "https://api.github.com/repos/ryantm/agenix/tarball/0.15.0",
+      "hash": "01dhrghwa7zw93cybvx4gnrskqk97b004nfxgsys0736823956la"
+    },
     "home-manager": {
       "type": "Git",
       "repository": {