Make job x86_64-gnu-debug run a subset of run-make tests

It looks like this job was intending to run all of the `needs-matching-clang`
tests (since they don't run without `RUSTBUILD_FORCE_CLANG_BASED_TESTS`), but
over time developed two problems:

- The tests it cares about were moved from run-make-fulldeps to run-make.
- Some of the relevant tests don't actually have "clang" in their name.

Switching to run-make solves the first problem, but we still don't run the
tests without "clang" in their name, because some of them are currently broken.

src/ci/docker/host-x86_64/x86_64-gnu-debug/Dockerfile

@@ -44,6 +44,14 @@ ENV RUST_CONFIGURE_ARGS \
       --set target.x86_64-unknown-linux-gnu.cc=clang \
       --set target.x86_64-unknown-linux-gnu.cxx=clang++
 
+# This job appears to be checking two separate things:
+# - That we can build the compiler with `--enable-debug`
+#   (without necessarily testing the result).
+# - That the tests with `//@ needs-matching-clang` pass, since they
+#   don't run by default unless RUSTBUILD_FORCE_CLANG_BASED_TESTS is set.
+#   - FIXME(https://github.com/rust-lang/rust/pull/126155#issuecomment-2156314273):
+#     Currently we only run the subset of tests with "clang" in their name.
+
 ENV SCRIPT \
   python3 ../x.py --stage 2 build && \
-  python3 ../x.py --stage 2 test tests/run-make-fulldeps --test-args clang
+  python3 ../x.py --stage 2 test tests/run-make --test-args clang