[build] Do not install rustup whether Rust is already installed

by the distribution package manager

Use 'command -v' to know if an executable is present in the PATH
There is an error with 'rustup' not found, but that's fine

neon/build.sh

 #!/bin/sh
 
 if [ -z "${DUNITER_FAST_BUILD}" ]; then
-    if [ -e "${HOME}/.cargo/bin/rustup" ]; then
+    if [ "$(command -v rustup)" ]; then
         rustup update stable
-    else
+    elif [ ! "$(command -v cargo)" ]; then
         curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
         export PATH="$HOME/.cargo/bin:$PATH"
     fi