ICU-21649 Adds build and execution of Unicode update tools to GHA CI.

Checks that the build process completes without failure and that the
generated data is identical with the data currently in the repository.

ICU-21649 Fix trigger mechanism.

.github/workflows/icu_ci.yml

@@ -399,9 +399,44 @@ jobs:
       - uses: actions/checkout@v2
       - run: python3 tools/scripts/uconfig_vars_test.py -u
 
-# Run header tests with UCONFIG_NO_XXX variations.
+  # Run header tests with UCONFIG_NO_XXX variations.
   uconfig_variation-check-all-header-tests:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
       - run: python3 tools/scripts/uconfig_vars_test.py -p
+
+  # Build Unicode update tools
+  unicode-update-tools:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: bazelbuild/setup-bazelisk@v1
+    - name: Mount bazel cache
+      uses: actions/cache@v2
+      with:
+        path: "~/.cache/bazel"
+        key: bazel
+
+    - name: Generate the data
+      run: |
+        export ICU_SRC=`pwd`;
+        icu4c/source/data/unidata/generate.sh;
+        if $?
+        then
+          echo
+          echo Build of Unicode update tools failed.
+          echo See
+          echo https://unicode-org.github.io/icu/processes/unicode-update#bazel-build-process
+          echo for how to reproduce and debug the failure
+          exit 1
+        fi
+
+        git diff --exit-code;
+        if $?
+        then
+          echo
+          echo ICU unicode data has changed!
+          echo Did you forget to include the changed data files in this PR?
+          exit 1
+        fi