Install dotnet-sdk-2.1 through apt-get (#17750)

The reason to make the change is to install a newer 2.1.x version, which support openssl 1.1.

Author: dolauli

tools/SwaggerCI/init.sh

@@ -5,14 +5,14 @@
 # Installed required packages
 sudo apt-get update \
     && sudo apt-get install -y curl \
+    && sudo apt-get install -y dotnet-sdk-2.1 \
     && curl -o- https://raw.githubusercontent.com/creationix/nvm/v0.33.11/install.sh | bash \
     && export NVM_DIR="$HOME/.nvm" \
     && [ -s "$NVM_DIR/nvm.sh" ] && \. "$NVM_DIR/nvm.sh" \
     && [ -s "$NVM_DIR/bash_completion" ] && \. "$NVM_DIR/bash_completion" \
     && nvm install 14.15.5 \
     && npm config set unsafe-perm true \
-    && npm install -g autorest \
-    && npm install -g dotnet-sdk-2.1
+    && npm install -g autorest
 
 # Write $PATH and some other envs to a file for later usage