ARROW-13027: [C++] Fix ASAN stack traces in CI

Before change:

```
Direct leak of 65536 byte(s) in 1 object(s) allocated from:
    #0 0x522f09 in
    #1 0x7f28ae5826f4 in
    #2 0x7f28ae57fa5d in
    #3 0x7f28ae58cb0f in
    #4 0x7f28ae58bda0 in
    ...
```

After change:
```
Direct leak of 65536 byte(s) in 1 object(s) allocated from:
    #0 0x522f09 in posix_memalign (/build/cpp/debug/arrow-dataset-file-csv-test+0x522f09)
    #1 0x7f28ae5826f4 in arrow::(anonymous namespace)::SystemAllocator::AllocateAligned(long, unsigned char**) /arrow/cpp/src/arrow/memory_pool.cc:213:24
    #2 0x7f28ae57fa5d in arrow::BaseMemoryPoolImpl<arrow::(anonymous namespace)::SystemAllocator>::Allocate(long, unsigned char**) /arrow/cpp/src/arrow/memory_pool.cc:405:5
    #3 0x7f28ae58cb0f in arrow::PoolBuffer::Reserve(long) /arrow/cpp/src/arrow/memory_pool.cc:717:9
    #4 0x7f28ae58bda0 in arrow::PoolBuffer::Resize(long, bool) /arrow/cpp/src/arrow/memory_pool.cc:741:7
    ...
```

Closes #10498 from westonpace/feature/ARROW-13027--c-fix-asan-stack-traces-in-ci

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>

ci/docker/ubuntu-20.04-cpp.dockerfile

@@ -127,6 +127,7 @@ ENV ARROW_BUILD_TESTS=ON \
     ARROW_WITH_SNAPPY=ON \
     ARROW_WITH_ZLIB=ON \
     ARROW_WITH_ZSTD=ON \
+    ASAN_SYMBOLIZER_PATH=/usr/lib/llvm-${llvm}/bin/llvm-symbolizer \
     AWSSDK_SOURCE=BUNDLED \
     GTest_SOURCE=BUNDLED \
     gRPC_SOURCE=BUNDLED \