ci: Add postsubmit task for hydrating proxy (#6047)

.github/workflows/postsubmit.yaml

@@ -0,0 +1,19 @@
+name: PostSubmit
+on:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
+jobs:
+  postsubmit:
+    if: github.repository == 'aws/karpenter-provider-aws'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ./.github/actions/install-deps
+      - name: hydrate-goproxy
+        run: |
+          mkdir -p hydrate-goproxy
+          cd hydrate-goproxy
+          go mod init hydrate-goproxy
+          go get "github.com/${GITHUB_REPOSITORY}@${GITHUB_SHA}"