Fix GitHub Actions config (actually run gdb tests on new backend).

I had missed that the CI config didn't actually run the tests, because
(I think) `matrix.target` is not set by default (?). All of our hosts
are native x86-64, so we can just gate on OS (Ubuntu) instead.

I also discovered that while I had been testing with the gdb tests
locally, when *all* `debug::*` tests are run, there are two that do not
pass on the new backend because of specific differences in compiled
code. One is a value-lifetime issue (the value is "optimized out" at the
point the breakpoint is set) and the other has to do with basic-block
order (it is trying to match against hardcoded machine-code offsets
which have changed).

.github/workflows/main.yml

@@ -295,8 +295,8 @@ jobs:
     # Test debug (DWARF) related functionality on new backend.
     - run: |
         sudo apt-get update && sudo apt-get install -y gdb
-        cargo test --features experimental_x64 test_debug_dwarf -- --ignored --test-threads 1
-      if: matrix.target == 'x86_64-unknown-linux-gnu'
+        cargo test --features experimental_x64 test_debug_dwarf -- --ignored --test-threads 1 --test debug::
+      if: matrix.os == 'ubuntu-latest'
       env:
         RUST_BACKTRACE: 1