This change aims to:
1) Remove our own `CCACHE_SIZE` environment variable that violates
Ccache's `CCACHE_*` namespace.
2) Introduce the `CCACHE_MAXSIZE` environment variable that is
documented since v3.3, which makes its usage consistent with other ones,
such as `CCACHE_DIR` and `CCACHE_NOHASHDIR`.
Also, fix a few bugs:
* Error: RPC command "enumeratesigners" not found in RPC_COMMANDS_SAFE_FOR_FUZZING or RPC_COMMANDS_NOT_SAFE_FOR_FUZZING. Please update test/fuzz/rpc.cpp.
* in run_once: ...format(" ".join(result.args), ... TypeError: sequence item 2: expected str instance, PosixPath found