V3 - Update via CP on shared hosting

Trying to update with CP I get the following error message

In Factory.php line 648:                                                                               
The HOME or COMPOSER_HOME environment variable must be set for composer to run correctly

I have seen this issue (https://statamic.com/forum/4834-composer-home-environment-variable-error-on-running-an-update) but I am on a shared hosting server, how can I use local composer.phar instead of global composer?

>>>>>>> Unanswered <<<<<<<
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