ci: add nix flake and use in CI

Using system tooling can lead to weird behavior of available Node
versions, for example this IPv6 resolution error:
```
[3/5] Fetching packages...
error Error: connect EHOSTUNREACH 2606:4700::6810:1922:443
    at TCPConnectWrap.afterConnect [as oncomplete] (node:net:1495:16)
```

Jenkinsfile

@@ -22,7 +22,9 @@ pipeline {
   stages {
     stage('Install') {
       steps {
-        sh 'yarn install'
+        script {
+          nix.develop('yarn install')
+        }
       }
     }
 
@@ -35,7 +37,7 @@ pipeline {
             variable: 'GITHUB_ACCESS_TOKEN'
           ),
         ]) {
-          sh 'yarn build'
+          nix.develop('yarn build')
         }
         jenkins.genBuildMetaJSON('build/build.json')
       } }
@@ -44,12 +46,13 @@ pipeline {
     stage('Publish') {
       steps {
         sshagent(credentials: ['status-im-auto-ssh']) {
-          sh """
+          nix.develop("""
             ghp-import \
               -b ${deployBranch()} \
               -c ${deployDomain()} \
               -p build
-          """
+          """,
+          pure: false)
         }
       }
     }

flake.lock

@@ -0,0 +1,26 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1753749649,
+        "narHash": "sha256-+jkEZxs7bfOKfBIk430K+tK9IvXlwzqQQnppC2ZKFj4=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "1f08a4df998e21f4e8be8fb6fbf61d11a1a5076a",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "ref": "nixos-25.05",
+        "type": "indirect"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,35 @@
+{
+  description = "Nix flake development shell.";
+
+  inputs = {
+    nixpkgs.url = "nixpkgs/nixos-25.05";
+  };
+
+  outputs =
+    { self, nixpkgs }:
+    let
+      supportedSystems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "x86_64-darwin"
+        "aarch64-darwin"
+      ];
+      forEachSystem = nixpkgs.lib.genAttrs supportedSystems;
+      pkgsFor = forEachSystem (system: import nixpkgs { inherit system; });
+    in
+    rec {
+      formatter = forEachSystem (system: pkgsFor.${system}.nixpkgs-fmt);
+
+      devShells = forEachSystem (system: {
+        default = pkgsFor.${system}.mkShellNoCC {
+          packages = with pkgsFor.${system}.buildPackages; [
+            git        # 2.44.1
+            openssh    # 9.7p1
+            yarn       # 1.22.22
+            nodejs_20  # v20.15.1
+            ghp-import # 2.1.0
+          ];
+        };
+      });
+    };
+}