Avoid some warnings-as-error in CircleCI+unity+AVX512F (#9978)

Summary:
Example failure when compiling on sufficiently new hardware and built-in headers:

```
In file included from /usr/local/lib/gcc/x86_64-linux-gnu/12.1.0/include/immintrin.h:49,
                 from ./util/bloom_impl.h:21,
                 from table/block_based/filter_policy.cc:31,
                 from unity.cc:167:
In function '__m512i _mm512_shuffle_epi32(__m512i, _MM_PERM_ENUM)',
    inlined from 'void XXH3_accumulate_512_avx512(void*, const void*, const void*)' at util/xxhash.h:3605:58,
    inlined from 'void XXH3_accumulate(xxh_u64*, const xxh_u8*, const xxh_u8*, size_t, XXH3_f_accumulate_512)' at util/xxhash.h:4229:17,
    inlined from 'void XXH3_hashLong_internal_loop(xxh_u64*, const xxh_u8*, size_t, const xxh_u8*, size_t, XXH3_f_accumulate_512, XXH3_f_scrambleAcc)' at util/xxhash.h:4251:24,
    inlined from 'XXH128_hash_t XXH3_hashLong_128b_internal(const void*, size_t, const xxh_u8*, size_t, XXH3_f_accumulate_512, XXH3_f_scrambleAcc)' at util/xxhash.h:5065:32,
    inlined from 'XXH128_hash_t XXH3_hashLong_128b_withSecret(const void*, size_t, XXH64_hash_t, const void*, size_t)' at util/xxhash.h:5104:39:
/usr/local/lib/gcc/x86_64-linux-gnu/12.1.0/include/avx512fintrin.h:4459:50: error: '__Y' may be used uninitialized [-Werror=maybe-uninitialized]
```

https://app.circleci.com/pipelines/github/facebook/rocksdb/13295/workflows/1695fb5c-40c1-423b-96b4-45107dc3012d/jobs/360416

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9978

Test Plan:
I was able to re-run in CircleCI with ssh, see the failure, ssh in and
verify that adding -fno-avx512f fixed the failure. Will watch build-linux-unity-and-headers

Reviewed By: riversand963

Differential Revision: D36296028

Pulled By: pdillinger

fbshipit-source-id: ba5955cf2ac730f57d1d18c2f517e92f34be77a3

.circleci/config.yml

@@ -413,6 +413,8 @@ jobs:
   build-linux-unity-and-headers:
     docker: # executor type
       - image: gcc:latest
+    environment:
+      EXTRA_CXXFLAGS: -mno-avx512f # Warnings-as-error in avx512fintrin.h, would be used on newer hardware
     resource_class: large
     steps:
       - checkout # check out the code in the project directory