musl/arm64: decompression before use the tarball.

In the last fix, the decompression ops is deleted by mistake and need
add it back.

Fixes: #1490
Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>

tools/osbuilder/scripts/lib.sh

@@ -285,6 +285,7 @@ ENV PATH=\$PATH:\$GOROOT/bin:\$GOPATH/bin
 RUN cd /tmp; \
 	mkdir -p /usr/local/musl/; \
 	if curl -sLO --fail https://musl.cc/${musl_tar}; then \
+		tar -zxf ${musl_tar}; \
 		cp -r ${musl_dir}/* /usr/local/musl/; \
 	else \
 		git clone ${musl_source_url}; \