chore: split coding standards on CI

[alexislefebvre]

composer validation and PHP-CS-Fixer will be run separately.

Before this change, an error with composer validate would stop the job before running PHP-CS-Fixer.

And it’s not necessary to install PHP to run composer validate, now the whole job takes about 5 seconds.

A fix for PHP-CS-Fixer is available here:

• fix: Authenticator must implement \Stringable #1755

The second commit will remove this output:

   0/168 [░░░░░░░░░░░░░░░░░░░░░░░░░░░░]   0%
  17/168 [▓▓░░░░░░░░░░░░░░░░░░░░░░░░░░]  10%
  34/168 [▓▓▓▓▓░░░░░░░░░░░░░░░░░░░░░░░]  20%
  51/168 [▓▓▓▓▓▓▓▓░░░░░░░░░░░░░░░░░░░░]  30%
  68/168 [▓▓▓▓▓▓▓▓▓▓▓░░░░░░░░░░░░░░░░░]  40%
  84/168 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓░░░░░░░░░░░░░░]  50%
 101/168 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓░░░░░░░░░░░░]  60%
 112/168 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓░░░░░░░░░░]  66%
 118/168 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓░░░░░░░░░]  70%
 133/168 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓░░░░░░]  79%
 152/168 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓░░░]  90%
 159/168 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓░░]  94%
 168/168 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%