patch 8.2.0306: Vim9: :substitute(pat(repl does not work in Vim9 script

Problem:    Vim9: :substitute(pat(repl does not work in Vim9 script.
Solution:   Remember starting with a colon. (closes #5676)
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 70c0da2..62cab6e 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -1647,6 +1647,9 @@
     int		save_reg_executing = reg_executing;
     int		ni;			// set when Not Implemented
     char_u	*cmd;
+#ifdef FEAT_EVAL
+    int		starts_with_colon;
+#endif
 
     vim_memset(&ea, 0, sizeof(ea));
     ea.line1 = 1;
@@ -1689,6 +1692,7 @@
     ea.cookie = cookie;
 #ifdef FEAT_EVAL
     ea.cstack = cstack;
+    starts_with_colon = *skipwhite(ea.cmd) == ':';
 #endif
     if (parse_command_modifiers(&ea, &errormsg, FALSE) == FAIL)
 	goto doend;
@@ -1713,7 +1717,7 @@
 	ea.cmd = skipwhite(ea.cmd + 1);
 
 #ifdef FEAT_EVAL
-    if (current_sctx.sc_version == SCRIPT_VERSION_VIM9)
+    if (current_sctx.sc_version == SCRIPT_VERSION_VIM9 && !starts_with_colon)
 	p = find_ex_command(&ea, NULL, lookup_scriptvar, NULL);
     else
 #endif