[c8 eol] move test rpm builds from c8 to c9

Summary:
Starting to see this intermittent c8 rpm build failure: P1610432199
https://fburl.com/scuba/buck2_action_errors/zjzzczdv

Since this is intermittent, it's causing false red signals on diffs (fails on
diff run and then succeeds on base run).

It only occurs when building rpms on c8. We're in the process of getting
rid of c8, so lets just move c8 rpm builds to c9.

Test Plan:
Did a build:
```
buck2 build \
  --show-full-output \
  --target-universe \
  fbcode//antlir/antlir2/features/facebook/chef_solo/tests/centos8:test-rpms \
  fbcode//antlir/antlir2/features/rpm/tests/repo:foo-empty-2-1.noarch--package
```

Then went to the repo build dir and verified that all the spec.json
files referenced the centos9 build appliance.

Reviewed By: naveedgol

Differential Revision: D63556190

fbshipit-source-id: d61f53749051ec4e5b2daa01c6c5b15f3be233d2

antlir/antlir2/bzl/configured_alias.bzl

@@ -90,7 +90,7 @@ _antlir2_configured_alias_macro = rule_with_default_target_platform(_configured_
 def antlir2_configured_alias(
         *,
         name: str,
-        default_os: str | None = None,
+        default_os: str | Select | None = None,
         rootless: bool | None = None,
         visibility: list[str] | None = None,
         **kwargs):

antlir/antlir2/features/rpm/tests/repo/BUCK

@@ -1,3 +1,4 @@
+load("//antlir/antlir2/bzl:configured_alias.bzl", "antlir2_configured_alias")
 load("//antlir/antlir2/bzl/feature:defs.bzl", "feature")
 load("//antlir/antlir2/bzl/image:defs.bzl", "image")
 load("//antlir/antlir2/package_managers/dnf/rules:repo.bzl", "repo")
@@ -223,7 +224,7 @@ all_rpms.append(
 )
 
 repo(
-    name = "test-repo",
+    name = "test-repo-impl",
     compress = "none",
     rpms = all_rpms,
     timestamp = 0,
@@ -238,3 +239,13 @@ repo(
         "//antlir/fbpkg/build/tw/tests:",
     ],
 )
+
+antlir2_configured_alias(
+    name = "test-repo",
+    actual = ":test-repo-impl",
+    #default_os = "centos9",
+    default_os = select({
+        "//antlir/antlir2/os:centos8": "centos9",
+        "DEFAULT": None,
+    }),
+)