add no-op mode for rustup script

When calling this script, it immediately starts the download process.
This change is mostly useful for when we want to source this file from
another script file.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

rustup-init.sh

@@ -808,4 +808,6 @@ get_strong_ciphersuites_for() {
     fi
 }
 
-main "$@" || exit 1
+if [ "$NO_OP" != "1" ]; then
+    main "$@" || exit 1
+fi