compiler: default to .whole cache mode for self-hosted backends

This is a better default cache mode until incremental compilation is
enabled by default.

Author: mlugg

src/main.zig

@@ -3470,7 +3470,11 @@ fn buildOutputType(
         // incremental cache mode is used for LLVM backend too.
         if (create_module.resolved_options.use_llvm) break :b .whole;
 
-        break :b .incremental;
+        // Eventually, this default should be `.incremental`. However, since incremental
+        // compilation is currently an opt-in feature, it makes a strictly worse default cache mode
+        // than `.whole`.
+        // https://github.com/ziglang/zig/issues/21165
+        break :b .whole;
     };
 
     process.raiseFileDescriptorLimit();