Fix RandomX staging: use install/fast to skip dependency rebuild

The 'make install' target triggers 'make all' which rebuilds the
benchmark binary (failing on Windows mingw). Use 'install/fast'
which is a cmake-generated target that installs without rebuilding.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: vrogojin

depends/packages/randomx.mk

@@ -18,5 +18,5 @@ define $(package)_build_cmds
 endef
 
 define $(package)_stage_cmds
-  $(MAKE) DESTDIR=$($(package)_staging_dir) install
+  $(MAKE) DESTDIR=$($(package)_staging_dir) install/fast
 endef