Increase nominal ccache cache size for coverage CI job

Speeding up builds for coverage reporting is desirable for two reasons:
1. Coverage-checking builds are approximately twice as slow compared to
release builds, even in absence of any ccache-supported caching on
either side.
2. Test runs with coverage logging and coverage collection take
approximately 46 minutes when regular test runs just take 17 minutes on
a similar platform.

Therefore, effective caching during builds has the potential to
considerably speed up the overall job. Caching using ccache, however,
was not effective, because ccache cleanups kicked in:cached artifacts
were being removed while the build was still in progress. The resulting
cache archive, therefore, was necessarily incomplete. ccache initiates a
cleanup when a subdirectory reaches max_size / 16. Increasing max_size
from 4 GB to 7 GB (empirically devised value) avoids such automatic
cleanup during builds while still keeping the overall cache size well
below 5 GB.

Author: tautschnig

.github/workflows/pull-request-checks.yaml

@@ -803,7 +803,7 @@ jobs:
       - name: Configure CMake CBMC build with coverage instrumentation parameters
         run: cmake -S . -Bbuild -Denable_coverage=1 -Dparallel_tests=2 -DCMAKE_CXX_COMPILER=/usr/bin/g++
       - name: Zero ccache stats and limit in size
-        run: ccache -z --max-size=4G
+        run: ccache -z --max-size=7G
       - name: Execute CMake CBMC build
         run: cmake --build build -- -j2
       - name: Print ccache stats