Build variables not working in post-processor

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Description

Currently, it's not possible to use the variable build.name in a post-processor but it's possible to use build.ID, or at least it doesn't throw an error.
The same goes for other variables on build like Host, User or Password.

Simple example:

source "null" "first-example" {
  communicator = "none"
}
build {
  name = "step1"
  sources = ["null.first-example"]
  provisioner "shell-local" {
    # Works
    inline = ["echo ${build.name}"]
  }
  post-processor "shell-local" {
    # Error: Unsupported attribute; This object does not have an attribute named "name".
    inline = ["echo ${build.name}"]
  }
}

Use Case(s)

My use case is the following, I would like to be able to use the build.name in the filename of the generated manifest.
This way I would only have to maintain one value at one place.

build {
  name = "step1"
  sources = ["null.first-example"]

  post-processor "manifest" {
    output = "manifests/${build.name}.${source.type}.${source.name}.json"
  }
}

Potential configuration

None

Potential References

The pull request which added the variable in provisioner: #10114

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