patch 8.2.4754: using cached values after unsetting some environment variables

Problem:    Still using cached values after unsetting some known environment
            variables.
Solution:   Take care of the side effects. (closes #10194)
diff --git a/src/evalvars.c b/src/evalvars.c
index 4690d1a..8f8df0c 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -1795,7 +1795,7 @@
 
 	// Environment variable, normal name or expanded name.
 	if (*lp->ll_name == '$')
-	    vim_unsetenv(lp->ll_name + 1);
+	    vim_unsetenv_ext(lp->ll_name + 1);
 	else if (do_unlet(lp->ll_name, forceit) == FAIL)
 	    ret = FAIL;
 	*name_end = cc;