Selenium suites: use RABBITMQ_ENABLED_PLUGINS over RABBITMQ_ENABLED_PLUGINS_FILE (backport #13752)

[mergify]

gmake was ignoring RABBITMQ_ENABLED_PLUGINS_FILE. The majority of the suites were working because all plugins were enabled. But only when I wanted to specify just a few it did not work. It was always enabling all plugins.
With this change, the content of the file enabled_plugins in selenium is converted into the value of the env var RABBITMQ_ENABLED_PLUGINS.

This change is necessary for tanzu wsr testing.

Types of Changes

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• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

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This is an automatic backport of pull request #13752 done by [Mergify](https://mergify.com).