utils: raise `PinnedVersion` to an argument
This is required so that we can use a custom pinned toolchain. The three
arguments required are:
- `PinnedBuild` (URL to toolchain)
- `PinnedSHA256` (SHA256 for installer)
- `PinnedVersion` (toolchain version)
Prior to this, we would error out as `PinnedVersion` would be unset and
could not be specified as it was not an argument.
