ci: Increase the timeout for extensive tests

The reorganization PR has caused this to fail once before because every
file shows up as changed. Increase the timeout so this doesn't happen.

We now cancel the job if too many extensive tests are run unless `ci:
allow-many-extensive` is in the PR description, so this helps prevent
the limit being hit by accident.

Author: tgross35

libm/.github/workflows/main.yaml

@@ -261,7 +261,7 @@ jobs:
       - clippy
       - calculate_extensive_matrix
     runs-on: ubuntu-24.04
-    timeout-minutes: 180
+    timeout-minutes: 240 # 4 hours
     strategy:
       matrix:
         # Use the output from `calculate_extensive_matrix` to calculate the matrix