patch 8.2.0284: Vim9: assignment test fails

Problem:    Vim9: assignment test fails.
Solution:   Avoid duplicating "s:".
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 7fe2764..08cf0e6 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -3393,7 +3393,7 @@
 		    generate_LOAD(cctx, ISN_LOADG, 0, name + 2, type);
 		    break;
 		case dest_script:
-		    compile_load_scriptvar(cctx, name);
+		    compile_load_scriptvar(cctx, name + (name[1] == ':' ? 2 : 0));
 		    break;
 		case dest_env:
 		    // Include $ in the name here