Fix relibc's recipe.toml

recipes/relibc/recipe.toml

+[recipe]
+# TODO: Set git link once userspace_fexec is merged.
+#GIT=https://gitlab.redox-os.org/redox-os/relibc.git
+
+[build]
+template = "custom"
+script = """
+cd "${COOKBOOK_SOURCE}"
+"$REDOX_MAKE" CARGO="env -u CARGO cargo" -j"$($NPROC)" DESTDIR="${COOKBOOK_STAGE}" install
+"""