Trailing slash redirect broken when using path prefix

[patrickdemers6]

Preliminary Checks

• This issue is not a duplicate. Before opening a new issue, please search existing issues: https://github.com/gatsbyjs/gatsby/issues
• This issue is not a question, feature request, RFC, or anything other than a bug report directly related to Gatsby. Please post those things in GitHub Discussions: https://github.com/gatsbyjs/gatsby/discussions

Description

A project using pathPrefix and trailingSlash does not redirect properly.

When a page is loaded and requires a redirect due to the slash, the redirect does not take pathPrefix into account.

Reproduction Link

https://github.com/patrickdemers6/gatsby-slash-redirect-path-prefix-bug

Steps to Reproduce

1. Set pathPrefix to anything.
2. Set trailingSlash to never.
3. Run with gatsby build --prefix-paths and gatsby serve --prefix-paths
4. Load a page with trailing slash.

Example: pathPrefix = /prefix

Expected Result

The redirect removes just the trailing slash.

Expected /prefix/hello/ => /prefix/hello

Actual Result

The redirect removes the slash and the pathPrefix.

Actual /prefix/hello/ => /hello

Environment

System:
    OS: macOS 14.5
    CPU: (12) arm64 Apple M2 Pro
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.17.0 - ~/.nvm/versions/node/v18.17.0/bin/node
    Yarn: 1.22.22 - ~/.nvm/versions/node/v18.17.0/bin/yarn
    npm: 9.6.7 - ~/.nvm/versions/node/v18.17.0/bin/npm
  Browsers:
    Chrome: 126.0.6478.127
    Safari: 17.5
  npmPackages:
    gatsby: ^5.14.0-next.4 => 5.14.0-next.4
  npmGlobalPackages:
    gatsby-cli: 5.13.3

Config Flags

No response