🔧👷 properly exclude build directory from CodeQL (#242)

## Description

This small PR adjusts the CodeQL config to ignore the `build` folder.
Since the introduction of using `FetchContent`, the build folder also
contains a `_deps` subfolder with all the fetched dependencies. These
might (and in the case of QECC here, will) trigger CodeQL warnings we
have no control over.
This PR should reduce the number of warnings flagged for the project.

## Checklist:

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- [x] The pull request only contains commits that are related to it.
- [x] I have added appropriate tests and documentation.
- [x] I have made sure that all CI jobs on GitHub pass.
- [x] The pull request introduces no new warnings and follows the
project's style guidelines.

Signed-off-by: burgholzer <[email protected]>

.github/workflows/reusable-code-ql.yml

@@ -65,6 +65,7 @@ jobs:
         with:
           patterns: |
             -**/extern/**
+            -**/build/**
           input: sarif-results/${{ matrix.language }}.sarif
           output: sarif-results/${{ matrix.language }}.sarif