nvm: load nvm script only if command doesn't already exist (#5454)

Fixes #5453.

plugins/nvm/nvm.plugin.zsh

 # Set NVM_DIR if it isn't already defined
 [[ -z "$NVM_DIR" ]] && export NVM_DIR="$HOME/.nvm"
 
-# Load nvm if it exists
-[[ -f "$NVM_DIR/nvm.sh" ]] && source "$NVM_DIR/nvm.sh"
+# Try to load nvm only if command not already available
+if ! type "nvm" &> /dev/null; then
+    # Load nvm if it exists
+    [[ -f "$NVM_DIR/nvm.sh" ]] && source "$NVM_DIR/nvm.sh"
+fi