[Chroot Builder] `"vm" template function is not defined` error

[JenGoldstrich]

This is a followup issue to: #87, the 'vm' template function defined in the chroot docs here does not work in the source block of the chroot builder, as far as I can tell this function has not worked for several years, in #87 this was causing an issue where in the builder here we try to use the interpolate library with the VM function which was failing, by moving where we register the function to after the Decode function we were able to fix the issues with the temporary values in #87 but the vm function is still unusable inside of a user template.

[lbajolet-hashicorp]

For context: the Decode function comes from the plugin SDK, and that issue of custom functions being unable to use for interpolation in the configs is present on all plugins (assuming they do define their own before calling Decode on the configuration), and has been since hashicorp/packer#9622, included in Packer 1.6.1 and onwards.