buildsys: drop variant arg for variant subcommands

[bcressey]

Issue number:
N/A

Description of changes:
Fixes the repack-variant task by using the common cargo_manifest_dir arg instead of the redundant variant arg.

Testing done:
Confirmed that build-variant, repack-variant, and build-all tasks worked.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[bcressey]

Previously, repack-variant would fail with this error:

[cargo-make][1] INFO - Running Task: repack-variant
error: the following required arguments were not provided:
  --variant <VARIANT>

Usage: buildsys repack-variant --name <NAME> --variant <VARIANT> --version-build <VERSION_BUILD> --version-image <VERSION_IMAGE> --image-dir <IMAGE_DIR> --arch <ARCH> --cargo-metadata-path <CARGO_METADATA_PATH> --root-dir <ROOT_DIR> --state-dir <STATE_DIR> --version-full <VERSION_FULL> --cargo-manifest-dir <CARGO_MANIFEST_DIR> --sdk-image <SDK_IMAGE> --tools-dir <TOOLS_DIR>

That's because the task invokes buildsys this way:

buildsys repack-variant --cargo-manifest-dir "variants/${BUILDSYS_VARIANT}"

But it doesn't also set CARGO_MANIFEST_DIR in the environment, so the variant arg for the subcommand wasn't getting set.

I opted to simplify the handling in buildsys rather than adjusting the behavior in Makefile.toml.