sessiond: disable clang-format to work around unstable output

When running format-cpp multiple times, I see clang-format-14
alternating between these two forms:

        _environment += lttng::format(
            "   {} = \"{}\";\n", field.name, escape_tsdl_env_string_value(field.value));

        _environment += lttng::format(" {} = \"{}\";\n",
                          field.name,
                          escape_tsdl_env_string_value(field.value));

Disable clang-format locally to avoid always having some spurious
changes.

Change-Id: I71b10a2ad1a5264f26c61f54743f298eb10917bf
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>