ci: Fix doxygen doc job (#798)

Pip changed it's dependency resolution algorithm a while ago. The job used an older version of pip which installed an inconsistent set of dependencies with a dependency update over the weekend. The job now updates pip first to get the new resolution behavior, which should fix the issue.

.github/workflows/builds.yml

@@ -164,7 +164,8 @@ jobs:
       - name: Install dependencies
         run: >
           apt-get install -y doxygen
-          && pip3 install -r docs/requirements.txt
+          && pip3 install --upgrade pip
+          && pip install -r docs/requirements.txt
       - name: Configure
         run: cmake -B build -S . -GNinja -DACTS_BUILD_DOCS=on
       - name: Build