fix(docker): disable glibc _FORTIFY_SOURCE for aarch64-musl cross builds

When cross-compiling aws-lc-sys for aarch64-unknown-linux-musl using
aarch64-linux-gnu-gcc, glibc's _FORTIFY_SOURCE generates calls to
__memcpy_chk, __fprintf_chk etc. which don't exist in musl, causing
linker errors. Disable it via CFLAGS_aarch64_unknown_linux_musl.

.github/workflows/container_latest.yml

@@ -92,6 +92,8 @@ jobs:
         run: |
           sudo apt-get install -y gcc-aarch64-linux-gnu
           echo "CARGO_TARGET_AARCH64_UNKNOWN_LINUX_MUSL_LINKER=aarch64-linux-gnu-gcc" >> "$GITHUB_ENV"
+          # Disable glibc fortify source — its __memcpy_chk etc. symbols don't exist in musl
+          echo "CFLAGS_aarch64_unknown_linux_musl=-U_FORTIFY_SOURCE" >> "$GITHUB_ENV"
 
       - name: Cache cargo registry and target
         uses: actions/cache@v5