fix: do not override user-defined THEIA_CONFIG_DIR

[xai]

What it does

The current behavior of Theia is to ignore a user-defined THEIA_CONFIG_DIR environment variable, which is an unexpected regression from a user's point of view.

This patch ensures that a user-defined value has precedence over the BackendApplicationPath/data/user-data location and the default location ~/.theia.

Fixes #13700

Contributed on behalf of STMicroelectronics

How to test

1. Set the THEIA_CONFIG_DIR environment variable to a non-default value: export THEIA_CONFIG_DIR=~/theia-test-data-dir
2. Start Theia (e.g., yarn browser start)
3. Confirm that you see a line similar to the following in stdout, indicating that the value you provided in step 1 is used as configuration directory:

   Configuration directory URI: 'file:///home/xai/theia-test-data-dir'

4. Confirm that this directory has been created on your filesystem (if it has not already existed before)

   Follow-ups

Review checklist

• [x] As an author, I have thoroughly tested my changes and carefully followed the review guidelines

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