gh-35632: Use fixed random-seed in the "Build & Test" CI
    
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### 📚 Description

Related to #35522 and #35544. Should at least fix the changed codecov
between runs.

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URL: #35632
Reported by: Tobias Diez
Reviewer(s): Matthias Köppe

Author: Release Manager

.github/workflows/build.yml

@@ -3,6 +3,9 @@ name: Build & Test
 on:
   pull_request:
   push:
+    branches: ['**']
+    # Ignore pushes on tags to prevent two uploads of codecov reports
+    tags-ignore: ['**']
   workflow_dispatch:
     # Allow to run manually
     inputs:
@@ -91,7 +94,7 @@ jobs:
         if: always() && steps.build.outcome == 'success'
         run: |
           ../sage -python -m pip install coverage
-          ../sage -python -m coverage run ./bin/sage-runtests --all -p2
+          ../sage -python -m coverage run ./bin/sage-runtests --all -p2 --random-seed=286735480429121101562228604801325644303
         working-directory: ./src
 
       - name: Prepare coverage results