ci: add a 'build-itest' job to main workflow

Adds a separate 'build-itest' job that builds the itest binaries prior
to the actual itest jobs being kicked off. With this change we'll
rebuild the itest binaries at most once, instead of at most four times.

.github/workflows/main.yml

@@ -267,12 +267,37 @@ jobs:
       - name: run ${{ matrix.unit_type }}
         run: make ${{ matrix.unit_type }}
 
+  ########################
+  # build itest binaries (cache warming)
+  ########################
+  build-itest:
+    name: build itest binaries
+    runs-on: ubuntu-latest
+    steps:
+      - name: git checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: setup go ${{ env.GO_VERSION }}
+        uses: ./.github/actions/setup-go
+        with:
+          go-version: '${{ env.GO_VERSION }}'
+          key-prefix: itest
+
+      - name: prepare dummy static data
+        run: mkdir -p app/build; touch app/build/index.html
+
+      - name: build itest binaries
+        run: make build-itest build-itest-binary
+
   ########################
   # integration tests
   ########################
   itest:
     name: integration test
     runs-on: ubuntu-latest
+    needs: build-itest
     strategy:
       # Allow other tests in the matrix to continue if one fails.
       fail-fast: false