feat: support local Bazel binaries using bazel_binaries.local()

[cgrindel]

Allow a client to execute Bazel integration tests using a local Bazel binary.

bazel_binaries = use_extension(
    "//:extensions.bzl",
    "bazel_binaries",
    dev_dependency = True,
)
bazel_binaries.download(version_file = "//:.bazelversion")
bazel_binaries.download(version = "6.4.0")
bazel_binaries.local(path = "/path/to/bazel", name = "foo"). # <=== An absolute or relative path to a Bazel binary.
use_repo(
    bazel_binaries,
    "bazel_binaries",
    # ...
    "build_bazel_bazel_foo",   # <=== The repository name for the local Bazel binary.
)

Then, you can use the Bazel binary in integration tests by using the name value as the version.

script_test(
    name = "local_bazel_binary_test",
    srcs = ["local_bazel_binary_test.sh"],
    bazel_binaries = bazel_binaries,
    bazel_version = "foo",  # <=== The version is the name specified above.
    deps = [
        "@bazel_tools//tools/bash/runfiles",
        "@cgrindel_bazel_starlib//shlib/lib:assertions",
    ],
)

[katre]

Looks good!