Variable substitution via shortcodes fails in `icon` and `href` fields of `about` links

Bug description

Suppose you have an item baz in _variables.yml, which contains named subitems foo and bar. If one uses the shortcode {{<var baz.foo >}} in the links subkey of an about page, such as:

title: About Page
about:
  links:
    - icon: {{<var baz.foo >}}
      href: {{<var baz.bar >}}

No variable substitution is performed.

This relates to #1627, #8987, and #4680.

Thank You!

Steps to reproduce

_variables.yml, in project root:

gh: 
  txt: GitHub
  lnk: https://github.com/
  icn: "github"
    
lnkdn:
  txt: Linkedin
  icn: "linkedin"
  lnk: "https://www.linkedin.com/in/"

about/index.qmd:

---
title: "More About Me"
image: ../imgs/bio-photo-1.jpg
sidebar: false
about:
  id: about-content
  template: trestles
  image-shape: round
  image-width: 250px
  links:
    - icon: github
      text: ""
      href: "{{< var gh.lnk >}}"
    - icon: linkedin
      text: "[]({{< var lnkdn.lnk >}})"
---

:::{#about-content}
<!-- stuff -->
:::

I've tried various flavors of the above links key based on similar issues. I've also tried this using an about.qmd in the root of the project. I am not able to get variable substitution to work as expected.

For completeness, here is a portion of my root _quarto.yml:

project:
  type: website

website:
  title: "Connor T. Wiegand"
  description: ""

  navbar:
    title: "Me"
    left:
      - text: About Me
        href: about/index.qmd

FWIW, adding repo-url: https://github.com/connortwiegand/ and replacing one of the link options with "{{< meta website.repo-url >}}" produces the following:

WARNING (/opt/quarto/share/filters/main.lua:17471) Unknown meta key website.repo-url specified in a metadata Shortcode.

Expected behavior

var shortcodes are substituted with their values from _variables.yml.

Actual behavior

No variable substitution is performed.

Your environment

• VSCode 1.91.1
• Linux Mint 21.3 (Ubuntu 22.04 -- Jammy)

Quarto check output

Quarto 1.5.52
[✓] 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: 1.5.52
      Path: /opt/quarto/bin

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

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /home/cwiegand/.TinyTeX/bin/x86_64-linux
      Version: 2024

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

[✓] Checking Python 3 installation....OK
      Version: 3.11.6 (Conda)
      Path: /home/cwiegand/miniconda3/bin/python
      Jupyter: 5.7.2
      Kernels: python3

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

[✓] Checking R installation...........OK
      Version: 4.4.1
      Path: /usr/lib/R
      LibPaths:
        - /home/cwiegand/R/x86_64-pc-linux-gnu-library/4.4
        - /usr/local/lib/R/site-library
        - /usr/lib/R/site-library
        - /usr/lib/R/library
      knitr: 1.48
      rmarkdown: 2.27

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

---

EDIT:

Variable substitution does work inside the text key. But not inside the icon or href keys. So this correctly injects a variable:

about:
  links
    - icon: github
      text: "{{< var gh.text >}}"

While both of these fail:

about:
  links
    - icon:  "{{< var gh.icon >}}"
      href: "{{< var gh.link >}}"