ci: Allow improper_ctypes for MSRV checks

The new extern type representation raises the lint prior to 1.72 because
ZSTs behind pointers were not allowed.

Author: tgross35

.github/workflows/ci.yaml

@@ -107,8 +107,10 @@ jobs:
         run: |
           set -eux
           if [ "${{ matrix.toolchain }}" = "1.65.0" ]; then
-              # Remove `-Dwarnings` at the MSRV since lints may be different
-              export RUSTFLAGS=""
+              # Remove `-Dwarnings` at the MSRV since lints may be different, and allow
+              # `improper_ctypes` since pointers to ZSTs got the warning prior to 1.72.
+              # FIXME(msrv): remove this flag when possible.
+              export RUSTFLAGS="-Aimproper_ctypes"
               # Remove `ctest` which uses the 2024 edition
               perl -i -ne 'print unless /"ctest(-test)?",/ || /"libc-test",/' Cargo.toml
           fi