Explicitly set php version to be used during CI build

[balsama]

Motivation
There were a couple of changes in shivammathur/setup-php (specifically #754) which resulted in new builds using PHP 8.0 during the build step.

Proposed changes
This change explicitly sets the php version to be used.

Alternatives considered
None

Testing steps
The build step will fail (example) without this with:

"Error: ] Your system is not ready to run the application.                       
                                                                                

Fix the following mandatory requirements:
=========================================

 * The application requires a version matching "^8.1"."

And it will pass with this PR merged.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (2691e56) 91.76% compared to head (e2f6b4e) 91.76%.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1595   +/-   ##
=========================================
  Coverage     91.76%   91.76%           
  Complexity     1809     1809           
=========================================
  Files           124      124           
  Lines          6471     6471           
=========================================
  Hits           5938     5938           
  Misses          533      533           

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[danepowell]

Thanks. I was initially confused by this report because Acquia CLI itself only requires PHP 8.0. But the Box compiler requires PHP 8.1; that's what was throwing the error when the setup-php action changed and started running PHP 8.0.

This seems like the best solution, short of a separate PHP version for require and require-dev, which is a whole other kettle of fish: composer/composer#5136