fix(snowflake): ensure that timestamps are properly timezoned

[cpcloud]

Fixes upstream snowflake test that was not properly converting pandas Timestamps using its preferred tz_* methods

[cpcloud]

Snowflake is passing:

cloud in 🌐 falcon in …/ibis on  more-timestamp-fixes is 📦 v9.5.0 via 🐍 v3.12.8 via ❄️  impure (ibis-3.12-env)
❯ pytest -m snowflake -n 8 --dist loadgroup -q
bringing up nodes...
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1818 passed, 10 skipped, 219 xfailed in 208.66s (0:03:28)