ci: also offer to test 3.12 with rust

ci: also offer to test 3.12 with rust

The rust-cpython binding got 3.12 support very recently, it is worse keeping on
a tighter watch.

--- a/contrib/heptapod-ci.yml	Wed Aug 28 16:35:43 2024 +0200
+++ b/contrib/heptapod-ci.yml	Wed Sep 11 00:20:07 2024 +0200
@@ -79,7 +79,7 @@
         RUNTEST_ARGS: "--pure --blacklist /tmp/check-tests.txt"
         TEST_HGMODULEPOLICY: "py"
 
-test-rust:
+test-rust: &test_rust
     <<: *runtests
     variables:
         HGWITHRUSTEXT: cpython
@@ -114,6 +114,12 @@
     variables:
         PYTHON: python3.12
 
+test-3.12-rust:
+    <<: *test_rust
+    when: manual  # avoid overloading the CI by default
+    variables:
+        PYTHON: python3.12
+
 check-pytype:
     extends: .runtests_template
     before_script: