fix: specify NODE_MAJOR as 20 [skip ci]

Author: aminya

dev/docker/setup-cpp-ubuntu-20.0.4.dockerfile

@@ -7,7 +7,7 @@ RUN apt-get update -qq && \
     apt-get install -y --no-install-recommends curl gnupg ca-certificates && \
     mkdir -p /etc/apt/keyrings && \
     curl -fsSL https://deb.nodesource.com/gpgkey/nodesource-repo.gpg.key | gpg --dearmor -o /etc/apt/keyrings/nodesource.gpg && \
-    echo "deb [signed-by=/etc/apt/keyrings/nodesource.gpg] https://deb.nodesource.com/node_$NODE_MAJOR.x nodistro main" | tee /etc/apt/sources.list.d/nodesource.list && \
+    echo "deb [signed-by=/etc/apt/keyrings/nodesource.gpg] https://deb.nodesource.com/node_20.x nodistro main" | tee /etc/apt/sources.list.d/nodesource.list && \
     apt-get update -qq && \
     apt-get install -y --no-install-recommends nodejs && \
     # install setup-cpp