set cache to readonly if skipping save

[planetmarshall]

If we aren't saving the cache we can get a small performance improvement by not writing new cache files during a build.

[hendrikmuhs]

Ccache documentation contains the following:

If true, ccache will attempt to use existing cached object files, but it will not to try to add anything new to the cache. If you are using this because your ccache directory is read-only, then you need to set temporary_dir as otherwise ccache will fail to create temporary files.

Can you check this?

Sorry, never mind. I misunderstood,it only applies if the whole directory is on a read-only filesystem or if the directory has read-only file permissions.