fix: restore UV_SYSTEM_PYTHON env var in TCK workflow

The uv pip install command fails without a virtual environment unless
UV_SYSTEM_PYTHON=1 is set to allow system-wide installation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jmesnil

.github/workflows/run-tck.yml

@@ -13,6 +13,8 @@ on:
 env:
   # Tag/branch of the TCK
   TCK_VERSION: 1.0-dev
+  # Tells uv to not need a venv, and instead use system
+  UV_SYSTEM_PYTHON: 1
   SUT_URL: http://localhost:9999
 
 # Only run the latest job
@@ -29,18 +31,26 @@ jobs:
     steps:
       - name: Checkout a2a-java
         uses: actions/checkout@v6
-      - name: Checkout a2a-tck
-        uses: actions/checkout@v6
-        with:
-          repository: a2aproject/a2a-tck
-          path: a2a-tck
-          ref: ${{ env.TCK_VERSION }}
       - name: Set up JDK ${{ matrix.java-version }}
         uses: actions/setup-java@v5
         with:
           java-version: ${{ matrix.java-version }}
           distribution: 'temurin'
           cache: maven
+      - name: Build a2a-java SDK
+        run: mvn -B install -DskipTests
+      - name: Extract a2a-java version
+        id: extract-version
+        run: |
+          A2A_JAVA_VERSION=$(mvn help:evaluate -Dexpression=project.version -q -DforceStdout)
+          echo "version=$A2A_JAVA_VERSION" >> $GITHUB_OUTPUT
+          echo "Detected a2a-java version: $A2A_JAVA_VERSION"
+      - name: Checkout a2a-tck
+        uses: actions/checkout@v6
+        with:
+          repository: a2aproject/a2a-tck
+          path: a2a-tck
+          ref: ${{ env.TCK_VERSION }}
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
@@ -50,14 +60,6 @@ jobs:
           pip install uv
           uv pip install -e .
         working-directory: a2a-tck
-      - name: Build a2a-java SDK
-        run: mvn -B install -DskipTests
-      - name: Extract a2a-java version
-        id: extract-version
-        run: |
-          A2A_JAVA_VERSION=$(mvn help:evaluate -Dexpression=project.version -q -DforceStdout)
-          echo "version=$A2A_JAVA_VERSION" >> $GITHUB_OUTPUT
-          echo "Detected a2a-java version: $A2A_JAVA_VERSION"
       - name: Generate a2a-java SUT
         run: A2A_JAVA_SDK_VERSION=${{ steps.extract-version.outputs.version }} make codegen-a2a-java-sut
         working-directory: a2a-tck