build: Mbed TLS should use MBEDTLS_ROOT first.

[gdamore]

The old MBEDTLS_ROOT_DIR is still honored for legacy compat, but the version 4.3 find script won't use it by default.

fixes #

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.16%. Comparing base (6a403d0) to head (380799e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1785 +/- ## ========================================== + Coverage 79.06% 79.16% +0.09% ========================================== Files 94 94 Lines 21077 21077 ========================================== + Hits 16665 16686 +21 + Misses 4412 4391 -21 ```

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