Build/Test Tools: Disable GitHub Action workflow runs triggered on `push` for forks and mirrors.

In the current state, the workflows run regardless of the repository context. This results many needless workflow runs that waste resources.

Workflow runs for private repositories are not free (accounts have a finite allotment of minutes for private repositories). This becomes problematic in private repositories that also mirror the WordPress develop repository, as any workflow runs will draw from a user’s or organization’s allotted action minutes.

Without blanket disabling the workflow manually for all event triggers, or modifying the workflow files in the forked/mirrored repository, there is no way to tune when the workflows run.

This change introduces a conditional statement into all GitHub Action workflows that prevents them from running on forked/mirrored repositories that are not `wordpress-develop`, except when a pull request is being submitted to that repository.

The exception to this is the Welcome workflow that posts a helpful message to first time contributors to `wordpress-develop`. This message is specific to this repository, so should only run when a PR is submitted to that repository.

See #50401.
Built from https://develop.svn.wordpress.org/trunk@49781


git-svn-id: http://core.svn.wordpress.org/trunk@49504 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.7-alpha-49780';
+$wp_version = '5.7-alpha-49781';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.