cli: fix attachment log and creation time CLI args (#1498)

### Changelog
- Fixed a bug where `mcap add attachment --log-time` would fail with a
parse error.
- Fixed a bug where `mcap add attachment --creation-time` would set the
wrong creation time.
### Docs

None.

### Description

Fixes two bugs in the parsing logic for `mcap add attachment` creation
time and log time.
<!-- Describe the problem, what has changed, and motivation behind those
changes. Pretend you are advocating for this change and the reader is
skeptical. -->

<!-- In addition to unit tests, describe any manual testing you did to
validate this change. -->

<table><tr><th>Before</th><th>After</th></tr><tr><td>

<!--before content goes here-->

</td><td>

<!--after content goes here-->

</td></tr></table>

<!-- If necessary, link relevant Linear or Github issues. Use `Fixes:
foxglove/repo#1234` to auto-close the Github issue or Fixes: FG-### for
Linear isses. -->


<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Fixes `mcap add attachment` to parse `--log-time` correctly and bind
`--creation-time` to the proper variable.
> 
> - **CLI**
>   - **Add Attachment** (`go/cli/mcap/cmd/attachment.go`):
> - Parse `--log-time` using `addAttachmentLogTime` (was using
creation-time var).
> - Bind `--creation-time` flag to `addAttachmentCreationTime` (was
bound to log-time var).
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
cdf15f7b2d54df83c13316f5196d3c01c0d6cab4. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->