minor #12650 Add note about .env.local being ignored in test environment (clemherreman)

This PR was merged into the 4.3 branch.

Discussion
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Add note about .env.local being ignored in test environment

This PR add a short note, warning developers that the `.env.local` is always ignored on test environments.

This surprised me, because I am using PHP VCR to isolate my test suite. I set my real API credential in a `.env.local`, however I was suprised to see they were not used when running my test suite against the real API.

I am not sure about which branch I should target, so I aimed at the one when you click on "Edit this page" on the documentation page.

Commits
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3a5c137 Add note about .env.local being ignored in test environment

Author: javiereguiluz

configuration.rst

@@ -627,6 +627,11 @@ the only mandatory file and each file content overrides the previous one:
   env vars overrides only for some environment. It's similar to ``.env.local``,
   but the overrides only apply to some particular environment.
 
+.. note::
+
+    ``.env.local`` is always ignored in test environment. The logic is that
+    tests should always produce the same results for everyone.
+
 .. note::
 
     The real environment variables defined in the server always win over the