refactor: move all datetime.now() calls to UTC (#1589)

# What does this PR do?

Updated all instances of datetime.now() to use timezone.utc for
consistency in handling time across different systems. This ensures that
timestamps are always in Coordinated Universal Time (UTC), avoiding
issues with time zone discrepancies and promoting uniformity in
time-related data.

Signed-off-by: Sébastien Han <seb@redhat.com>

llama_stack/providers/inline/tool_runtime/code_interpreter/code_execution.py

@@ -168,7 +168,7 @@ def process_matplotlib_response(response, matplotlib_dump_dir: str):
     image_paths = []
     for i, img in enumerate(images):
         # create new directory for each day to better organize data:
-        dump_dname = datetime.today().strftime("%Y-%m-%d")
+        dump_dname = datetime.today().strftime("%Y-%m-%d")  # noqa: DTZ002 - we don't care about timezones here since we are displaying the date
         dump_dpath = Path(matplotlib_dump_dir, dump_dname)
         dump_dpath.mkdir(parents=True, exist_ok=True)
         # save image into a file