MINOR: [CI][C++] Show machine info in Linux GHA jobs (#49736)

### Rationale for this change

Detailed info about available memory and CPU model can help in certain cases, for example to know whether a certain instruction set is available (example in #47769).

### Are these changes tested?

N/A.

### Are there any user-facing changes?

No.

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>

Author: pitrou

.github/workflows/cpp.yml

@@ -109,6 +109,10 @@ jobs:
       LLVM: ${{ matrix.llvm }}
       UBUNTU: ${{ matrix.ubuntu }}
     steps:
+      - name: Show machine info
+        run: |
+          free -h
+          lscpu
       - name: Checkout Arrow
         uses: actions/checkout@v6
         with: