run_binary expands $(locations …) to a single string instead of a list

Example:

WORKSPACE:

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

http_archive(
name = "bazel_skylib",
sha256 = "fc64d71583f383157e3e5317d24e789f942bc83c76fde7e5981cadc097a3c3cc",
strip_prefix = "bazel-skylib-1.1.1/",
urls = [
"https://github.com/bazelbuild/bazel-skylib/archive/refs/tags/1.1.1.zip", # 2021-09-27
],
)
```

BUILD:

```python
load("@bazel_skylib//rules:run_binary.bzl", "run_binary")

sh_binary(
name = "cat",
srcs = ["cat.sh"],
)

filegroup(
name = "files",
srcs = [
"a.txt",
"b.txt",
],
)

run_binary(
name = "run_cat",
srcs = [":files"],
outs = ["cat.out"],
args = [
"$(location :cat.out)",
"$(locations :files)",
],
tool = ":cat",
)
```

cat.sh:

```bash
#!/bin/bash
out="$1"
shift
cat -- "$@" > "${out:?}"
```

a.txt and b.txt are arbitrary source files.

* * *

Running `bazel build //:cat.out` will then result in an error message:

```
cat: './a.txt ./b.txt': No such file or directory
```

This is because `$(locations …)` expands to a single string.

Not sure whether this can be fixed in Skylib alone – probably needs a Bazel/Starlark change for a variant of `ctx.expand_location` that works on an `Args` object.

Reproduce the issue with the WORKSPACE and BUILD example, using `bazel build //:cat.out` and `cat.sh`. Read `run_binary.bzl` and investigate the reported `ctx.expand_location` and `Args` behavior in Bazel/Starlark. Done means `$(locations :files)` reaches `cat.sh` as separate arguments so both input files are read successfully.

Written by the indexing model from the issue text.