Refactor pull_request_number method into a constant

No need to read every time

Author: mokagio

fastlane/Fastfile

@@ -780,7 +780,7 @@ platform :ios do
       Branch: `#{ENV.fetch('BUILDKITE_BRANCH', 'N/A')}`
       Commit: #{ENV.fetch('BUILDKITE_COMMIT', 'N/A')[0...7]}
     NOTES
-    release_notes += "Pull Request: ##{pull_request_number}" unless pull_request_number.nil?
+    release_notes += "Pull Request: ##{PULL_REQUEST_NUMBER}" unless PULL_REQUEST_NUMBER.nil?
 
     firebase_app_distribution(
       app: FIREBASE_APP_ID,
@@ -796,13 +796,13 @@ platform :ios do
       dsym_path: './build/WooCommerce.app.dSYM.zip'
     )
 
-    next if pull_request_number.nil?
+    next if PULL_REQUEST_NUMBER.nil?
 
     # PR Comment
     comment_body = prototype_build_details_comment(app_display_name: 'WooCommerce iOS Prototype')
     comment_on_pr(
       project: GITHUB_REPO,
-      pr_number: pull_request_number,
+      pr_number: PULL_REQUEST_NUMBER,
       reuse_identifier: 'prototype-build-link',
       body: comment_body
     )
@@ -812,7 +812,7 @@ platform :ios do
   lane :build_for_prototype_build do |fetch_code_signing: true|
     update_certs_and_profiles_enterprise if fetch_code_signing
 
-    pr_prefix = pull_request_number&.then { |num| "pr#{num}" }
+    pr_prefix = PULL_REQUEST_NUMBER&.then { |num| "pr#{num}" }
     commit_short_hash = ENV.fetch('BUILDKITE_COMMIT', '0')[0...7]
     build_number = [pr_prefix, commit_short_hash].compact.join('-')
 
@@ -1526,11 +1526,8 @@ TEST_ANALYTICS_ENVIRONMENT = %w[
 # Release Management Utils
 # -----------------------------------------------------------------------------------
 
-def pull_request_number
-  # Buildkite sets this env var to the PR number if on a PR, but to 'false' (and not nil) if not on a PR
-  pr_num = ENV.fetch('BUILDKITE_PULL_REQUEST', 'false')
-  pr_num == 'false' ? nil : Integer(pr_num)
-end
+# Buildkite sets this env var to the PR number if on a PR, but to 'false' (and not nil) if not on a PR
+PULL_REQUEST_NUMBER = ENV['BUILDKITE_PULL_REQUEST']&.then { |n| n == 'false' ? nil : Integer(n) }
 
 def create_backmerge_pr
   version = release_version_current