fromAbsoluteFilePath: not an absolute path when running cabal update on Windows in Github Actions with built-in ghcup

**Describe the bug**
I'm getting the following error when running `cabal update` in very specific circumstances.

```
fromAbsoluteFilePath: not an absolute path
CallStack (from HasCallStack):
error, called at src\\Hackage\\Security\\Util\\Path.hs:297:31 in hackage-secur_-0.6.0.1-aa3b50000d0dcf2987b9cef364fe5c110fa92705:Hackage.Security.Util.Path
```

**To Reproduce**
Steps to reproduce the behavior:

For this repository: https://github.com/input-output-hk/cardano-node

Checkout `always-use-ghcup`. Create a new PR with the latest commit. (You may need to amend the commit to force a build).

I've not seen the error outside of this environment.

**Expected behavior**
The `cabal update` command should work.

**System information**
- Operating system: Windows on Github Actions
- `cabal-3.6.2.0`, `ghc-8.10.7` versions

**Additional context**
Related PRs and tickets:

* https://github.com/haskell/hackage-security/pull/261
* https://gitlab.haskell.org/haskell/ghcup-hs/-/issues/308