[CICD-892] Add wpe-update-source-selector mu plugin to exclude list

[mike-day]

JIRA Ticket

CICD-892

What Are We Doing Here

A new mu-plugin should be ignored in site deploys. We can do so by adding the plugin dir to our excludes script.

Testing

Unit

Ran the make target and confirmed all unit tests pass with the updated fixtures.

Manual

In a local test site, I ran through the following steps:

• Created an mu-plugin under a new wpe-update-source-selector dir
• Changed dirs to wp-content (problematic for mu-plugin excludes in the past)
• Created this .env file (note the --dry-run flag):
  
• Using the new image, ran the dry run deploy and noted the file list:
  
• Updated the .env to remove the --dry-run flag
• Ran a real deploy, confirming the file list still excluded the new mu-plugin