Replace non existing "build_tar" local tool with a repo target

[mindaugasrukas]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

$ cd testing/default_toolchain/

$ bazel cquery --override_repository=io_bazel_rules_docker=/home/user/bazelbuild/rules_docker '@docker_config//:*'
INFO: Invocation ID: b42e2f4d-afa1-4269-8594-21b2babb178e
INFO: Repository build_tar instantiated at:
  /home/user/bazelbuild/rules_docker/testing/default_toolchain/WORKSPACE:29:11: in <toplevel>
Repository rule local_tool defined at:
  /home/user/bazelbuild/rules_docker/testing/default_toolchain/local_tool.bzl:37:29: in <toplevel>
ERROR: An error occurred during the fetch of repository 'build_tar':
   Traceback (most recent call last):
    File "/home/user/bazelbuild/rules_docker/testing/default_toolchain/local_tool.bzl", line 27, column 17, in _local_tool
        rctx.symlink(realpath, "bin/%s" % rctx.name)
Error in symlink: in call to symlink(), parameter 'from' got value of type 'NoneType', want 'string, Label, or path'
ERROR: Error fetching repository: Traceback (most recent call last):
    File "/home/user/bazelbuild/rules_docker/testing/default_toolchain/local_tool.bzl", line 27, column 17, in _local_tool
        rctx.symlink(realpath, "bin/%s" % rctx.name)
Error in symlink: in call to symlink(), parameter 'from' got value of type 'NoneType', want 'string, Label, or path'
ERROR: no such package '@build_tar//': in call to symlink(), parameter 'from' got value of type 'NoneType', want 'string, Label, or path'
INFO: Elapsed time: 0.333s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (0 packages loaded)
    Fetching @docker_config; Restarting.

Issue Number: #1997

What is the new behavior?

$ cd testing/default_toolchain/

$ bazel cquery --override_repository=io_bazel_rules_docker=/home/user/bazelbuild/rules_docker '@docker_config//:*'
INFO: Invocation ID: 07b7cace-09ed-4117-9ffd-645668ea03ff
INFO: Analyzed 2 targets (24 packages loaded, 149 targets configured).
INFO: Found 2 targets...
@docker_config//:BUILD (null)
@docker_config//:toolchain (50c9e17)
INFO: Elapsed time: 1.701s
INFO: 0 processes.
INFO: Build completed successfully, 0 total actions

Does this PR introduce a breaking change?

• Yes
• No