sysutils/installwatch: use BINARY_ALIAS to invoke ld.bfd

This port does not honour LD or -fuse-ld in CFLAGS via LLD_UNSAFE but
BINARY_ALIAS can be used to ensure we use the BFD linker.

PR: 226989
Approved by: portmgr (LLD_UNSAFE blanket)
Sponsored by: The FreeBSD Foundation