refactor(s390x): export rustc-symlink overlay + add formatter; refresh pins

Lift the s390x rustc symlink_file patch overlay into overlays.s390xRustcSymlink
so nixos-ci consumes one definition (patch travels with it) instead of
duplicating it. Add nixfmt-rfc-style formatter. Refresh fleet-pins + parity
inputs to HEAD so the lock collapses to a single nixpkgs (1c3fe55).

flake.lock

@@ -161,11 +161,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1779533061,
-        "narHash": "sha256-orWNYXtYURhEj3X4+xGMAhaEcKRvwXqTtJ8x2jV/M+Q=",
+        "lastModified": 1780368078,
+        "narHash": "sha256-tLzA5XveUF4PfuKNz3KuhmVhuME3PX5zvtFa17hhQPU=",
         "ref": "refs/heads/main",
-        "rev": "b818e345ec4470e4b3e335bd2f864183c512116d",
-        "revCount": 13,
+        "rev": "1626405d46ff3595b91c9e2d3ed9399f67c18b83",
+        "revCount": 15,
         "type": "git",
         "url": "https://git.oleks.space/oleks/fleet-pins"
       },
@@ -201,6 +201,10 @@
           "fleet-pins",
           "nixpkgs"
         ],
+        "nixpkgs-mermaid-gpu": [
+          "fleet-pins",
+          "nixpkgs"
+        ],
         "nixpkgs-micron": [
           "fleet-pins",
           "nixpkgs"
@@ -211,11 +215,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1776894566,
-        "narHash": "sha256-oD0s9bTy8piFts/RcZhibytVoSu42/mItsvxH2icGMc=",
+        "lastModified": 1780368078,
+        "narHash": "sha256-tLzA5XveUF4PfuKNz3KuhmVhuME3PX5zvtFa17hhQPU=",
         "ref": "main",
-        "rev": "1550e5be0e89a6a1b084a5662ad37b35b6905e49",
-        "revCount": 10,
+        "rev": "1626405d46ff3595b91c9e2d3ed9399f67c18b83",
+        "revCount": 15,
         "type": "git",
         "url": "https://git.oleks.space/oleks/fleet-pins"
       },
@@ -287,17 +291,17 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1771848320,
-        "narHash": "sha256-ppefbuGh9S1W3gNCvMPhsadn6FvshqaJAYRk1/N2JGY=",
+        "lastModified": 1777268161,
+        "narHash": "sha256-bxrdOn8SCOv8tN4JbTF/TXq7kjo9ag4M+C8yzzIRYbE=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "2fc6539b481e1d2569f25f8799236694180c0993",
+        "rev": "1c3fe55ad329cbcb28471bb30f05c9827f724c76",
         "type": "github"
       },
       "original": {
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "2fc6539b481e1d2569f25f8799236694180c0993",
+        "rev": "1c3fe55ad329cbcb28471bb30f05c9827f724c76",
         "type": "github"
       }
     },
@@ -328,11 +332,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1780472546,
-        "narHash": "sha256-cx+821qtyNZNe9t3ab8qImmeg/rxVzPpZIS45SflrI0=",
+        "lastModified": 1780602315,
+        "narHash": "sha256-msVp9pBMP+I/n0+GGADG1XQembM3F19fUIh79rxqkgs=",
         "ref": "refs/heads/main",
-        "rev": "d265a79ddb84b297364e6cc3638c9f6b5dc583d7",
-        "revCount": 10,
+        "rev": "089bd03264d848f8391d9d53d79da0fd97ed1857",
+        "revCount": 13,
         "type": "git",
         "url": "https://git.oleks.space/oleks/parity-lib"
       },