chown to buildslave the linaro download folder

Besides the Linaro platform binaries, several files are copy and look
up under this folder, thus enable buildslave permission.

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: Ifb091fb1cbff92718d953cebf9519af7634ad067
diff --git a/bionic-amd64-tf-a-build/Dockerfile b/bionic-amd64-tf-a-build/Dockerfile
index 0bad2eb..1a9ded6 100644
--- a/bionic-amd64-tf-a-build/Dockerfile
+++ b/bionic-amd64-tf-a-build/Dockerfile
@@ -79,7 +79,7 @@
     bash -ex /tmp/tf-dependencies.install ;\
     bash -ex /tmp/tf-environment.install ;\
     # Fix permissions
-    chown -R buildslave:buildslave ${TOOLS_DIR} /usr/share/plantuml ;\
+    chown -R buildslave:buildslave ${TOOLS_DIR} /usr/share/plantuml /nfs/downloads/linaro ;\
     # Cleanup
     apt clean ;\
     rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*