fix: UDP Exporter Sample App not starting via Gradle (#1171)

*Issue #, if available:*
- Sample app for UDP Exporter is failing via `gradle build`:
https://github.com/aws-observability/aws-otel-java-instrumentation/actions/runs/17167485805/job/48710900750

*Description of changes:*
- UDP e2e test uses `gradle` from GH Action env, which isn't stable. Use
`gradlew` that exists in this repo instead.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.

Author: jj22ee

.github/workflows/udp-exporter-e2e-test.yml

@@ -51,8 +51,8 @@ jobs:
         run: |
           export XRAY_UDP_SPAN_EXPORTER_VERSION=${{ steps.build-udp-exporter.outputs.xrayUdpSpanExporterVersion }}
           echo "Running Sample App against X-Ray UDP Span Exporter version: $XRAY_UDP_SPAN_EXPORTER_VERSION"
-          gradle build
-          gradle bootRun &
+          ../../gradlew build
+          ../../gradlew bootRun &
           sleep 5
 
       - name: Call Sample App Endpoint