PIpelines builds fail with "Command validate:all is not defined"

[ba66e77]

Describe the bug
Pipelines builds fail as a result of the validate:all command no longer existing. The same line exists in the Travis file as well, so we'd probably see the same thing there.

To Reproduce
Trigger a Pipelines build using the default acquia-pipelines.yml file, or otherwise invoke the vendor/acquia/blt/scripts/pipelines/validate script.

Expected behavior
The build should only fail because I broke things.

Detailed error output
The Pipelines logs show the error below. Since the

+ source ${BLT_DIR}/scripts/pipelines/validate

export PATH=${COMPOSER_BIN}:${PATH}

blt validate:all --define drush.alias='${drush.aliases.ci}' --ansi --verbose

In Application.php line 669:
                                                                  
  [Symfony\Component\Console\Exception\CommandNotFoundException]  

  Command "validate:all" is not defined.                          
                                                                  
  Did you mean one of these?                                      
      artifact:install:drupal                                     
      drupal:install                                              
      sync:all                                                    
      sync:all:db                                                 
     validate:composer                                           
      validate:php                                                
      validate:twig                                               
      validate:twig:lint:files                                    
      validate:yaml                                               
      validate:yaml:lint:files                                    

System information

• BLT version: 12.0.0

[ba66e77]

At a glance, changing the command to validate:all in this line seems reasonable: https://github.com/acquia/blt/blob/12.x/src/Robo/Commands/Validate/ValidateAllCommand.php#L15

However, that throws the validation into an infinite loop where it invokes everything in the namespace, including itself.

Changing the build files to run blt validate .... does seem to work as expected.