CI: Trigger Windows build on PR label 'windows'

Our CI matrix now includes Windows if the 'windows' label is added to
the PR. The job still takes too long to include it for every PR, but it
is certainly useful to have it run on Windows-specific PRs and PRs that
might break Windows builds.

.github/workflows/ci.yml

@@ -1,6 +1,13 @@
 name: CI
 
-on: [push, pull_request]
+on:
+  push:
+  pull_request:
+    types:
+      - opened
+      - synchronize
+      - reopened
+      - labeled
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.head_ref || format('{0}-{1}', github.ref, github.run_number) }}
@@ -9,7 +16,9 @@ concurrency:
 jobs:
   # CI matrix - runs the job in lagom-template.yml with different configurations.
   Lagom:
-    if: github.repository == 'LadybirdBrowser/ladybird'
+    if: |
+      github.repository == 'LadybirdBrowser/ladybird'
+      && (github.event_name != 'pull_request' || github.event.action != 'labeled' || github.event.label.name == 'windows')
     name: ${{ matrix.os_name }}, ${{ matrix.arch }}, ${{ matrix.build_preset }}, ${{ matrix.toolchain }}
 
     strategy:
@@ -44,6 +53,17 @@ jobs:
             clang_plugins: false
             runner_labels: '["blacksmith-16vcpu-ubuntu-2404"]'
 
+          - os_name: 'Windows'
+            arch: 'x86_64'
+            build_preset: 'Windows_Sanitizer_CI'
+            toolchain: 'ClangCL'
+            clang_plugins: false
+            runner_labels: '["windows-2025"]'
+
+        exclude:
+          # Exclude the Windows build from the matrix if the PR is not labeled with 'windows'.
+          - os_name: ${{ !contains(github.event.pull_request.labels.*.name, 'windows') && 'Windows' }}
+
     uses: ./.github/workflows/lagom-template.yml
     with:
       toolchain: ${{ matrix.toolchain }}