Fix sourcing of asdf.sh

Installing asdf via Homebrew now says:

```
To use asdf, add the following line to your ~/.zshrc:
  . /opt/homebrew/opt/asdf/libexec/asdf.sh
```

Yes, the laptop script is doing:

`. /opt/homebrew/opt/asdf/asdf.sh`

This used to work, but in no longer does because of the change in
Homebrew/homebrew-core#81664

This corrects the file location being source.

This failure was showing in CI and UTM locally for me.

mac

@@ -152,7 +152,7 @@ brew link --force heroku
 fancy_echo "Configuring asdf version manager ..."
 if [ ! -d "$HOME/.asdf" ]; then
   brew install asdf
-  append_to_zshrc "source $(brew --prefix asdf)/asdf.sh" 1
+  append_to_zshrc "source $(brew --prefix asdf)/libexec/asdf.sh" 1
 fi
 
 alias install_asdf_plugin=add_or_update_asdf_plugin
@@ -168,7 +168,7 @@ add_or_update_asdf_plugin() {
 }
 
 # shellcheck disable=SC1091
-sh "$(brew --prefix asdf)/asdf.sh"
+sh "$(brew --prefix asdf)/libexec/asdf.sh"
 asdfShExitCode=$?
 if [ $asdfShExitCode -ne 0 ]; then
   exit $asdfShExitCode