Try to fix libstdc++-not-found error

See https://github.com/diffblue/cbmc/actions/runs/6856483568/job/18643716705?pr=6749
https://github.com/rust-lang/cc-rs/blob/2d6a3b2119cf5eacc01e1f2877e064a7aede7819/src/lib.rs#L3497C52-L3497C76

Author: tautschnig

.github/workflows/pull-request-check-rust-api.yaml

@@ -102,6 +102,7 @@ jobs:
       # by the other jobs already present in `pull-request-checks.yaml`.
       - name: Run Rust API tests
         run: |
+          export MACOSX_DEPLOYMENT_TARGET=10.15
           VERSION=$(cat src/config.inc | python3 -c "import sys,re;line = [line for line in sys.stdin if re.search(r'CBMC_VERSION = (\d+\.\d+\.\d+)', line)];sys.stdout.write(re.search(r'CBMC_VERSION = (\d+\.\d+\.\d+)', line[0]).group(1))")
           cd src/libcprover-rust;\
           cargo clean;\