Ignore .zig-cache for new zig versions

zig cache directory was updated in ziglang/zig#20115

.gitignore

@@ -1,5 +1,8 @@
-/zig-cache/
+/.zig-cache/
 /zig-out/
 /answers/
 /patches/healed/
 /output/
+
+# Leave this in here for older zig versions
+/zig-cache/