Add ccache env variable

[adrian-cojocaru]

Currently ccache is added to the build without any checks -DANDROID_CCACHE=ccache and is expected to be present in the environment (for render-test/benchmark targets).
MapLibreAndroid checks a list of hardcoded paths for it to be enabled:

                        if (file("/usr/bin/ccache").exists()) {
                            arguments "-DANDROID_CCACHE=/usr/bin/ccache"
                        } else if (file("/usr/local/bin/ccache").exists()) {
                            arguments "-DANDROID_CCACHE=/usr/local/bin/ccache"
                        }

An environmental variable can be useful to configure the path (in addition to the existing path list).