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

@@ -1,5 +1,5 @@
 #!/usr/bin/env groovy
-library 'status-jenkins-lib@v1.8.15'
+library 'status-jenkins-lib@v1.9.24'
 
 pipeline {
   agent { label 'linux' }
@@ -23,32 +23,41 @@ pipeline {
   stages {
     stage('Install') {
       steps {
-        sh 'yarn install'
+        script {
+          nix.develop('yarn install')
+        }
       }
     }
 
     stage('Scrape') {
       steps {
-        sh 'yarn scrape'
+        script {
+          nix.develop('yarn scrape', keepEnv: ['INCLUDED_DIRS','FETCH_MODE'])
+        }
       }
     }
 
     stage('Build') {
-      steps { script {
-        sh 'yarn build'
-        jenkins.genBuildMetaJSON('build/build.json')
-      } }
+      steps {
+        script {
+          nix.develop('yarn build')
+          jenkins.genBuildMetaJSON('build/build.json')
+        }
+      }
     }
 
     stage('Publish') {
       steps {
         sshagent(credentials: ['status-im-auto-ssh']) {
-          sh """
-          ghp-import \
-              -b ${deployBranch()} \
-              -c ${deployDomain()} \
-              -p build
-          """
+          script {
+            nix.develop("""
+              ghp-import \
+                -b ${deployBranch()} \
+                -c ${deployDomain()} \
+                -p build
+            """, sandbox: false,
+                 keepEnv: ['SSH_AUTH_SOCK'])
+          }
         }
       }
     }