Increase nominal ccache cache size for coverage CI job

[tautschnig]

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.

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