build: add --unlink-first to tar command while extracting Golang

While extracting the Golang archive, `tar` errors out with `Operation
not permitted` when it changes permissions for directories. The
`--unlink-first` option removes the files first, so that no change in
mode is needed.

Signed-off-by: Niels de Vos <ndevos@ibm.com>

scripts/Dockerfile.test

@@ -50,7 +50,7 @@ RUN source /build.env \
     && gem install mdl \
     && mkdir -p ${GOROOT} \
     && curl https://storage.googleapis.com/golang/go${GOLANG_VERSION}.linux-${GOARCH}.tar.gz  \
-       | tar xzf - -C ${GOROOT} --strip-components=1 \
+       | tar xzf - -C ${GOROOT} --strip-components=1 --unlink-first \
     && curl -sf "https://raw.githubusercontent.com/golangci/golangci-lint/${GOLANGCI_VERSION}/install.sh" \
        | bash -s -- -b ${GOPATH}/bin "${GOLANGCI_VERSION}" \
     && curl -L "${HELM_SCRIPT}" | bash -s -- --version "${HELM_VERSION}" \