Build of maprule tests via "bazel build @bazel_skylib//tests/..." fails

WORKSPACE:

```
workspace(name = "foo")

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

skylib_version = "0.8.0"
http_archive(
name = "bazel_skylib",
type = "tar.gz",
url = "https://github.com/bazelbuild/bazel-skylib/releases/download/{}/bazel-skylib.{}.tar.gz".format (skylib_version, skylib_version),
sha256 = "2ef429f5d7ce7111263289644d233707dba35e39696377ebab8b0bc701f7818e",
)

load("@bazel_skylib//:workspace.bzl", "bazel_skylib_workspace")

bazel_skylib_workspace()
```

Command:

`bazel build @bazel_skylib//tests/...`

Output:
```
INFO: Analyzed 91 targets (0 packages loaded, 0 targets configured).
INFO: Found 91 targets...
ERROR: /usr/local/google/home/fwe/.cache/bazel/_bazel_fwe/ea0f404d46c6e8c3c74ad70e292ab53d/external/bazel_skylib/tests/maprule/BUILD:27:1: error executing shell command: '/bin/bash -c $MAPRULE_TOOL' failed (Exit 1) bash failed: error executing command /bin/bash -c '$MAPRULE_TOOL'

Use --sandbox_debug to see verbose messages from the sandbox
cat: ../bazel_skylib/tests/maprule/foo.txt: No such file or directory
INFO: Elapsed time: 0.440s, Critical Path: 0.12s
INFO: 3 processes: 3 linux-sandbox.
FAILED: Build did NOT complete successfully
```

Recreate the failure with the provided WORKSPACE and `bazel build @bazel_skylib//tests/...`. Start with `tests/maprule/BUILD` and the reported missing `tests/maprule/foo.txt`; done means the maprule tests build successfully without the missing-file error.

Written by the indexing model from the issue text.