[antlir2][centos10] use rpm2extents/rpmcow

Summary: This exists now, it will greatly speed up image builds

Test Plan:
Layer builds now use CoW rpm installation
```
❯ buck build fbcode//antlir/antlir2/facebook/images/build_appliance/centos10:build-appliance.impl.layer
Buck UI: https://www.internalfb.com/buck2/5cb36f63-0e24-486f-8ba0-dfe96f638619
Time elapsed: 37.6s
BUILD SUCCEEDED

❯ buck2 log what-ran --trace-id 5cb36f63-0e24-486f-8ba0-dfe96f638619 | grep rpm2extents | wc -l
Showing commands from: /data/users/vmagro/fbsource/tools/buck2-versions/stable-users build --show-output fbcode//antlir/antlir2/facebook/images/build_appliance/centos10:build-appliance.impl.layer
227
```

Reviewed By: naveedgol

Differential Revision: D67607966

fbshipit-source-id: dac9ff0d215fd230dfc5dfcf937668fc031af642

antlir/antlir2/bzl/dnf/reflink.bzl

@@ -6,6 +6,7 @@
 REFLINK_FLAVORS = {
     # @oss-disable
     # @oss-disable
+    # @oss-disable
 }
 
 def rpm2extents(