Loader does not decode pathPrefix correctly

[herecydev]

Description

Supplying an unencoded pathPrefix, such as /امتحان fails validation with an error message: "pathPrefix" must be a valid relative uri.

So one can supply something like pathPrefix: "/" + encodeURI("امتحان"), which solves that problem but then the app is not functional:

The issue looks to stem from these lines of code: https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby/cache-dir/find-path.js#L9-L11

The matching decodes the pathname uri, but because the path_prefix is not decoded it will fail to match.

Steps to reproduce

1. Clone https://github.com/herecydev/encoding-path-prefix-bug
2. Run yarn build
3. Run yarn serve
4. Navigate to http://localhost:9000/%D8%A7%D9%85%D8%AA%D8%AD%D8%A7%D9%86/

Expected result

Either the config should allow an unencoded pathPrefix or loader can correctly encode/decode when it needs to

Actual result

Gatsby loader fails to decode the pathPrefix

Environment

  System:
    OS: Windows 10 10.0.19043
    CPU: (32) x64 AMD Ryzen 9 5950X 16-Core Processor            
  Binaries:
    Node: 14.15.4 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.10 - ~\AppData\Roaming\npm\yarn.CMD
    npm: 6.14.10 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: Spartan (44.19041.906.0), Chromium (89.0.774.63)
  npmPackages:
    gatsby: ^3.1.2 => 3.1.2 
    gatsby-plugin-gatsby-cloud: ^2.1.0 => 2.1.0 
    gatsby-plugin-image: ^1.1.2 => 1.1.2 
    gatsby-plugin-manifest: ^3.1.0 => 3.1.0 
    gatsby-plugin-offline: ^4.1.0 => 4.1.0 
    gatsby-plugin-react-helmet: ^4.1.0 => 4.1.0 
    gatsby-plugin-sharp: ^3.1.2 => 3.1.2 
    gatsby-source-filesystem: ^3.1.0 => 3.1.0 
    gatsby-transformer-sharp: ^3.1.0 => 3.1.0