ci: Use documented CCACHE_MAXSIZE instead of CCACHE_SIZE

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`.

ci/test/06_script_b.sh

@@ -75,7 +75,7 @@ if [ -z "$NO_WERROR" ]; then
   BITCOIN_CONFIG_ALL="${BITCOIN_CONFIG_ALL} --enable-werror"
 fi
 
-ccache --zero-stats --max-size="${CCACHE_SIZE}"
+ccache --zero-stats
 PRINT_CCACHE_STATISTICS="ccache --version | head -n 1 && ccache --show-stats"
 
 if [ -n "$ANDROID_TOOLS_URL" ]; then