Follow dev doc guidance on when GitHub Actions should run

This utilises the guidance introduced in: alphagov/govuk-developer-docs#3961

.github/workflows/ci.yml

@@ -1,4 +1,14 @@
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+  workflow_dispatch:
+    inputs:
+      ref:
+        description: 'The branch, tag or SHA to checkout'
+        default: main
+        type: string
 
 jobs:
   snyk-security:
@@ -11,7 +21,7 @@ jobs:
       contents: read
       security-events: write
       actions: read
-  
+
   codeql-sast:
     name: CodeQL SAST scan
     uses: alphagov/govuk-infrastructure/.github/workflows/codeql-analysis.yml@main
@@ -21,7 +31,7 @@ jobs:
   dependency-review:
     name: Dependency Review scan
     uses: alphagov/govuk-infrastructure/.github/workflows/dependency-review.yml@main
-  
+
   # This matrix job runs the test suite against multiple Ruby and Rails versions
   test_matrix:
     strategy:
@@ -36,6 +46,8 @@ jobs:
       BUNDLE_GEMFILE: gemfiles/${{ matrix.gemfile }}.gemfile
     steps:
       - uses: actions/checkout@v4
+        with:
+          ref: ${{ inputs.ref || github.ref }}
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}