pre-bootstrap hook doesn't support bash scripts on Windows

We’re trying to make use of the pre-bootstrap hook functionality (on version `3.33.3`) but have hit an interesting issue which we wanted to raise:

## Background:
- We have an agent hooks directory, which we populate with bash scripts to perform our required actions.
Files are named such as:
- `pre-checkout`
- `pre-bootstrap`
- We for the most part (aside from some templating/minor tweaks) use the same hooks across different OSs (Windows, Linux in this case)
- This has worked fine for us so far
- `pre-bootstrap` hooks on Linux works fine

## Issue:
- On Windows agents, the `pre-bootstrap` fails with an error like: `Error: Error building command: exec: "c:\\users\\buildkite-agent\\AppData\\Local\\buildkite-agent\\hooks\\pre-bootstrap": file does not exist`
- We’ve done some digging and it seems that there might be a slightly different code path that gets hit for `pre-bootstrap`.
- `pre-bootstrap` calls `RunWithoutPrompt` [here](https://github.com/buildkite/agent/blob/86d12d8721054d710b13381c0f9d4109dd187a15/agent/job_runner.go#L648)
- Following the function calls, we get to [here](https://github.com/buildkite/agent/blob/86d12d8721054d710b13381c0f9d4109dd187a15/bootstrap/shell/shell.go#L149) where the `PATHEXT` env var is read.
- We don’t do anything special to this env var, so it is the Windows Server 2019 default of `.COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.MSC`
- We end up in the LookPath function [here](https://github.com/buildkite/agent/blob/86d12d8721054d710b13381c0f9d4109dd187a15/bootstrap/shell/lookpath_windows.go#L59)
- This checks for executables with one of the extensions above.
- This fails in our case as we have a file called `pre-bootstrap` (bash script) which has none of those extensions.
- Interestingly, it seems the other hooks take a slightly different approach
- They seem to enter executeHook [here](https://github.com/buildkite/agent/blob/86d12d8/bootstrap/bootstrap.go#L344)
- And then into [ScriptWrapper](https://github.com/buildkite/agent/blob/86d12d8/hook/scriptwrapper.go) which seems to handle bash hooks fine.

Hopefully I haven't missed anything. The behaviour took us a bit by surprise, we’d like to have our pre-bootstrap hook script be a bash script that can be used across the platforms.

Start in agent/job_runner.go at the pre-bootstrap RunWithoutPrompt call, then trace bootstrap/shell/shell.go and bootstrap/shell/lookpath_windows.go where PATHEXT is used. Compare this path with bootstrap/bootstrap.go's executeHook and hook/scriptwrapper.go. Done means a bash pre-bootstrap hook without a Windows executable extension runs on Windows while existing hook behavior remains intact.

Written by the indexing model from the issue text.