workspace_status_command cannot live in root of workspace

### Description of the problem / feature request:

If a script exists in the root of a workspace, it can't be used as a `--workspace_status_command` value.

If specifying `--workspace_status_command=foo.sh` then `foo.sh` will be looked up in `$PATH` but not the working directory.

If specifying `--workspace_status_command=./foo.sh` then [the option](https://github.com/bazelbuild/bazel/blob/8d2c45c5aabb3fceb67e433e89895883e9228daf/src/main/java/com/google/devtools/build/lib/analysis/WorkspaceStatusAction.java#L78-L90) is normalized using the `PathFragmentConverter` to `foo.sh` and the same behaviour is observed.

I would expect at least the second, if not both of the above commands, to work. Perhaps there's some behaviour possible where the working directory is searched _before_ `$PATH`? Alternatively, if we treated the option as a String, that would at least make the `./foo.sh` version work.

### Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

```console
% touch WORKSPACE
% cat > BUILD < status.sh <: BazelWorkspaceStatusAction stable-status.txt failed: Failed to determine workspace status: Process exited with status 127
/bin/sh: status.sh: command not found
Target //:gr failed to build
Use --verbose_failures to see the command lines of failed build steps.
```

### What operating system are you running Bazel on?

macOS

### What's the output of `bazel info release`?

release 5.0.0

Start with src/main/java/com/google/devtools/build/lib/analysis/WorkspaceStatusAction.java and the referenced PathFragmentConverter behavior. Reproduce the issue with the provided WORKSPACE, BUILD, and status.sh example using --workspace_status_command=./status.sh, then verify that the command resolves from the workspace root and the build produces FOO 1.

Written by the indexing model from the issue text.