Update asdf plugins already installed (#554)

This may resolve some issues updating the languages, especially nodejs
where the asdf plugin maintains the signing keys required to verify the
package.

In order not to break anyone's laptop.local scripts, keep an alias of
`install_asdf_plugin` to the new method.

mac

@@ -159,19 +159,22 @@ if [ ! -d "$HOME/.asdf" ]; then
   append_to_zshrc "source $HOME/.asdf/asdf.sh" 1
 fi
 
-install_asdf_plugin() {
+alias install_asdf_plugin=add_or_update_asdf_plugin
+add_or_update_asdf_plugin() {
   local name="$1"
   local url="$2"
 
   if ! asdf plugin-list | grep -Fq "$name"; then
     asdf plugin-add "$name" "$url"
+  else
+    asdf plugin-update "$name"
   fi
 }
 
 # shellcheck disable=SC1090
 source "$HOME/.asdf/asdf.sh"
-install_asdf_plugin "ruby" "https://github.com/asdf-vm/asdf-ruby.git"
-install_asdf_plugin "nodejs" "https://github.com/asdf-vm/asdf-nodejs.git"
+add_or_update_asdf_plugin "ruby" "https://github.com/asdf-vm/asdf-ruby.git"
+add_or_update_asdf_plugin "nodejs" "https://github.com/asdf-vm/asdf-nodejs.git"
 
 install_asdf_language() {
   local language="$1"