refactor(authz): use lock only during the variables update

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 64.62%. Comparing base (8fffa93) to head (e40ddac). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/server/authz/engine/rego/engine.go	75.00%	2 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3431 +/- ## ========================================== - Coverage 64.62% 64.62% -0.01% ========================================== Files 174 174 Lines 13907 13914 +7 ========================================== + Hits 8988 8992 +4 - Misses 4229 4231 +2 - Partials 690 691 +1 ``` | [Flag](https://app.codecov.io/gh/flipt-io/flipt/pull/3431/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flipt-io) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/flipt-io/flipt/pull/3431/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flipt-io) | `64.62% <75.00%> (-0.01%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=flipt-io#carryforward-flags-in-the-pull-request-comment) to find out more.

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