`labgrid/cfg/unstore.bzl` transition should take into account cmd line `--platforms` argument

unstore transition sets configuration //command_line_option:platforms to the value of //labgrid/cfg:platform label setting:

def implementation(settings, attr):
    platform = settings["//labgrid/cfg:platform"]
    return {
        "//command_line_option:platforms": str(platform),
    }

thus it does not care about previous //command_line_option:platforms's value.

This way every target which rule sets that configuration transition to, e.g. //labgrid/config/deps.bzl:deps

load("//labgrid/cfg:unstore.bzl", _cfg = "unstore")

deps = rule(
    doc = DOC,
    attrs = ATTRS,
    implementation = implementation,
    provides = [DefaultInfo, PyInfo],
    toolchains = ["//labgrid/toolchain/config:type"],
    cfg = _cfg,
)

will ignore command line --platform argument (setting it to value of //labgrid/cfg:platform).

--platform command line argument should be taken into account for bazel invocations.