Resolving a BuildpackReference created from a URL-like String can fail on Windows

[wilkinsona]

TarGzipBuildpack.resolve(this.resolverContext, reference) returns null on a GitHub Actions Windows runner as the Path -> String -> Path round-tripping does not work. I think we're seeing a difference compared to the Concourse Windows runner due to the build being run from a different working directory. On GitHub Actions, the value of the reference is something like file://C:\Users\RUNNER~1\AppData\Local\Temp\junit364266182815735792\buildpack\buildpack.tgz