build: fix test certificate chain generation

Author: nielsdevreede-rl

tests/unit/CMakeLists.txt

@@ -594,9 +594,11 @@ function(seastar_add_certgen name)
     DEPENDS "${CMAKE_CURRENT_BINARY_DIR}/${CERT_CERT}"
     WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
   )
-  file(COPY_FILE "${CMAKE_CURRENT_BINARY_DIR}/${CERT_CERT}" "${CMAKE_CURRENT_BINARY_DIR}/${CERT_CHAIN}")
-  file(READ "${CMAKE_CURRENT_BINARY_DIR}/${CERT_CAROOT}" CONTENTS)
-  file(APPEND "${CMAKE_CURRENT_BINARY_DIR}/${CERT_CHAIN}" "${CONTENTS}")
+  add_custom_command(OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/${CERT_CHAIN}"
+    COMMAND ${CMAKE_COMMAND} -E cat "${CMAKE_CURRENT_BINARY_DIR}/${CERT_CERT}" "${CMAKE_CURRENT_BINARY_DIR}/${CERT_CAROOT}" > "${CMAKE_CURRENT_BINARY_DIR}/${CERT_CHAIN}"
+    DEPENDS "${CMAKE_CURRENT_BINARY_DIR}/${CERT_CERT}" "${CMAKE_CURRENT_BINARY_DIR}/${CERT_CAROOT}"
+    WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+  )
 
   add_custom_target(${name}
     DEPENDS "${CMAKE_CURRENT_BINARY_DIR}/${CERT_CAROOT_DER}" "${CMAKE_CURRENT_BINARY_DIR}/${CERT_CERT_DER}" "${CMAKE_CURRENT_BINARY_DIR}/${CERT_CHAIN}"