build: append JOB_POOLS definition instead of setting it

in 18f43a0, a job definition is
added for serialization the calls to openssl commands. it sets
the global property named "JOB_POOLS". assuming we have a parent
project which uses Seastar. if:

- it is also using CMake and include Seastar as a subdirectory in its
  CMake building system, and
- it is also using "JOB_POOLS", and
- it sets this variable before Seastar does. this variable's value
  will be overwritten by Seastar. and all of the targets using the
  pool(s) defined by it won't be able to find the defined pool(s)
  anymore.

the typical error message would look like

```
CMake Error:
  Running

   '/home/kefu/.local/bin/ninja' '-C' '/home/kefu/dev/scylladb/build/cmake-test' '-t' 'recompact'

  failed with:

   ninja: error: build.ninja:1360: unknown pool name 'link_pool'
```

where CMake is able to create the building system. but ninja is not able
to find the definition of the pool.

so, in order to address this problem, in this changes

1. move the command which sets the JOB_POOLS property closer to
   where this pool is used.
2. instead setting the property, add "openssl_serial=1" to
   to it. so the existing value of this property is not overwritten.

since `seastar_gen_mtls_certs()` is only called once, and is not
supposed to be reused. we don't need to worry about failures like:

```
   ninja: error: CMakeFiles/rules.ninja:26: duplicate pool 'openssl_serial'
```

Signed-off-by: Kefu Chai <[email protected]>

tests/unit/CMakeLists.txt

@@ -526,9 +526,6 @@ function(seastar_add_certgen name)
 
   find_program(OPENSSL openssl)
 
-  # Some openssl commands read and write the serial number file (.srl). These can't be executed in parallel.
-  set_property(GLOBAL PROPERTY JOB_POOLS openssl_serial=1)
-
   add_custom_command(OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/${CERT_PRIVKEY}"
     COMMAND ${OPENSSL} genpkey -quiet -out ${CERT_PRIVKEY} -algorithm ${CERT_ALG} ${CERT_ALG_OPTS}
     WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
@@ -569,6 +566,9 @@ function(seastar_gen_mtls_certs)
   set(CLIENT1_SUBJECT "/C=GB/ST=London/L=London/O=Redpanda Data/OU=Core/CN=client1.org")
   set(CLIENT2_SUBJECT "/C=GB/ST=London/L=London/O=Redpanda Data/OU=Core/CN=client2.org")
 
+  # Some openssl commands read and write the serial number file (.srl). These can't be executed in parallel.
+  set_property(GLOBAL APPEND PROPERTY JOB_POOLS openssl_serial=1)
+
   add_custom_command(OUTPUT mtls_ca.key
     COMMAND ${OPENSSL} ecparam -name prime256v1 -genkey -noout -out mtls_ca.key
     WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}