gha: Use spdx3-validate to validate documents

This validator correctly handles ExternalMap references

.github/workflows/pull_request.yml

@@ -48,9 +48,7 @@ jobs:
       - name: Setup Python tools
         run: |
           python3 -m pip install -U pip
-          python3 -m pip install \
-            check-jsonschema \
-            pyshacl
+          python3 -m pip install spdx3-validate
 
       - name: Validate SPDX 2.2 & SPDX 2.3 Documents
         run: |
@@ -60,11 +58,7 @@ jobs:
 
       - name: Validate SPDX 3.0 Documents
         run: |
-          SPDX30_SCHEMA_URL="https://spdx.org/schema/3.0.0/spdx-json-schema.json"
-          SPDX30_SHACL_URL="https://spdx.org/rdf/3.0.0/spdx-model.ttl"
-
           for f in $(find . -type f -path '*/spdx-3.0/*.json'); do
               echo "Checking $f..."
-              check-jsonschema -v --schemafile $SPDX30_SCHEMA_URL $f
-              pyshacl -s $SPDX30_SHACL_URL -e $SPDX30_SHACL_URL $f
+              spdx3-validate -j $f
           done