COMP: Cache ExternalData and build against ITK main

Add GitHub Actions cache for ExternalData object stores in all three
build jobs (build-test-cxx, build-test-python-superbuild,
build-test-documentation). This avoids re-downloading 563 CID-addressed
test data files on every CI run.

Also update itk-git-tag from v6.0a02 to main to pick up CI fixes
merged after that tag (ccache, dashboard script, etc.).

Author: hjmjohnson

.github/workflows/build-test-publish.yml

@@ -3,7 +3,7 @@ name: Build, test, publish
 on: [push,pull_request]
 
 env:
-  itk-git-tag: "v6.0a02"
+  itk-git-tag: "main"
 
 jobs:
   build-test-cxx:
@@ -57,6 +57,14 @@ jobs:
           cd ITK
           git checkout ${{ env.itk-git-tag }}
 
+      - name: Restore ExternalData cache
+        uses: actions/cache@v4
+        with:
+          path: ${{ github.workspace }}/../bld/ExternalData/Objects
+          key: externaldata-v1-${{ hashFiles('Ex/src/**/*.cid') }}
+          restore-keys: |
+            externaldata-v1-
+
       - name: Build ITK
         if: matrix.os != 'windows-2022'
         run: |
@@ -169,6 +177,14 @@ jobs:
       - name: Get specific version of CMake, Ninja
         uses: lukka/get-cmake@v3.24.2
 
+      - name: Restore ExternalData cache
+        uses: actions/cache@v4
+        with:
+          path: ${{ github.workspace }}/../bld/ExternalData/Objects
+          key: externaldata-superbuild-v1-${{ matrix.os }}-${{ hashFiles('Ex/src/**/*.cid') }}
+          restore-keys: |
+            externaldata-superbuild-v1-${{ matrix.os }}-
+
       - name: Fetch CTest driver script
         run: |
           curl -L https://raw.githubusercontent.com/InsightSoftwareConsortium/ITKSphinxExamples/dashboard/itkexamples_common.cmake -O
@@ -308,6 +324,14 @@ jobs:
           SITE_PACKAGES_DIR=$(python3 "-c" "from distutils import sysconfig; print(sysconfig.get_python_lib())")
           sed -i "6559d" ${SITE_PACKAGES_DIR}/sphinx/domains/cpp.py
 
+      - name: Restore ExternalData cache
+        uses: actions/cache@v4
+        with:
+          path: ${{ github.workspace }}/../bld/ExternalData/Objects
+          key: externaldata-docs-v1-${{ hashFiles('Ex/src/**/*.cid') }}
+          restore-keys: |
+            externaldata-docs-v1-
+
       - name: Fetch CTest driver script
         run: |
           curl -L https://raw.githubusercontent.com/InsightSoftwareConsortium/ITKSphinxExamples/dashboard/itkexamples_common.cmake -O