email+revealjs overwrites slides HTML

Bug description

Rendering a document containing two formats (revealjs and email) overwrites the first HTML file with the second format. In this example, we want the email-preview/ and .output_metadata.json but do not want the HTML file it produces.

Steps to reproduce

---
title: email and slides
format:
  revealjs: {}
  email: {}
---

## important

Your analysis might be best presented by using a slide show.

::: {.email}

::: {.subject}
email from slides
:::

It may be important to email key metrics.

:::

Expected behavior

A revealjs document is allowed to produce slides HTML alongside email metadata.

Actual behavior

A vanilla HTML document overwrites the slides HTML and also produces email metadata.

Your environment

No response

Quarto check output

Quarto 1.5.57
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.2.0: OK
      Dart Sass version 1.70.0: OK
      Deno version 1.41.0: OK
      Typst version 0.11.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.5.57
      Path: /Users/aron/quarto/quarto-1.5.57-macos/bin

[✓] Checking tools....................OK
      TinyTeX: (not installed)
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: Installation From Path
      Path: /Library/TeX/texbin
      Version: 2024

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.12.2
      Path: /opt/homebrew/opt/python@3.12/bin/python3.12
      Jupyter: (None)

      Jupyter is not available in this Python installation.
      Install with python3 -m pip install jupyter

[✓] Checking R installation...........OK
      Version: 4.3.3
      Path: /Library/Frameworks/R.framework/Resources
      LibPaths:
        - /Library/Frameworks/R.framework/Versions/4.3-arm64/Resources/library
      knitr: 1.48
      rmarkdown: 2.27

[✓] Checking Knitr engine render......OK

Start by rendering the markdown example with both revealjs and email formats, then inspect the rendering path that handles their outputs. Done means the revealjs slides HTML remains intact while email-preview/ and .output_metadata.json are produced without an unwanted vanilla HTML overwrite.