Mk/bsd.ccache.mk: Fix Poudriere CCACHE_STATIC_PREFIX feature.

Poudriere's CCACHE_STATIC_PREFIX feature (not Mk/) avoids installing
ccache for each port using the native jail ABI and instead copies in a
host static binary. It uses NO_CCACHE_DEPEND to avoid installing
ccache. For example, this is used for ccache-memcached where the
dependency chain would make using ccache unfeasible.

Poudriere sets CCACHE_WRAPPER_PATH, rather than the new CCACHE_PKG_PREFIX.
Because CCACHE_BIN is now computed from CCACHE_PKG_PREFIX rather than
CCACHE_WRAPPER_PATH, CCACHE_BIN was incorrect. Now compute
CCACHE_PKG_PREFIX from CCACHE_WRAPPER_PATH if it is set, otherwise
fallback to using LOCALBASE and computing CCACHE_BIN and
CCACHE_WRAPPER_PATH from there.

PR 257151
Fixes: 6b641f4eb4 ("Mk: Document and simply user-modifiable ...")