patch 8.0.1554: custom plugins loaded with --clean

Problem:    Custom plugins loaded with --clean.
Solution:   Do not include the home directory in 'runtimepath'.
diff --git a/src/os_amiga.h b/src/os_amiga.h
index 5154e28..336cbdb 100644
--- a/src/os_amiga.h
+++ b/src/os_amiga.h
@@ -43,6 +43,9 @@
 #ifndef	DFLT_RUNTIMEPATH
 # define DFLT_RUNTIMEPATH "home:vimfiles,$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after,home:vimfiles/after"
 #endif
+#ifndef	CLEAN_RUNTIMEPATH
+# define CLEAN_RUNTIMEPATH "$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after"
+#endif
 
 #ifndef	BASENAMELEN
 # define BASENAMELEN	26	/* Amiga */