ci: fix reference to coryan's fork (#3634)

Used my fork to debug the build and forgot to revert the change
when sending the original PR.

Author: coryan

ci/kokoro/docker/Dockerfile.ubuntu-install

@@ -156,7 +156,7 @@ RUN ldconfig
 
 # Download and compile google-cloud-cpp from source too:
 WORKDIR /var/tmp/build
-RUN wget -q https://github.com/coryan/google-cloud-cpp-common/archive/v0.25.0.tar.gz && \
+RUN wget -q https://github.com/googleapis/google-cloud-cpp-common/archive/v0.25.0.tar.gz && \
     tar -xf v0.25.0.tar.gz && \
     cd /var/tmp/build/google-cloud-cpp-common-0.25.0 && \
     cmake -H. -Bcmake-out \