fake-package + --package-db=relative/path fails

**Describe the bug**

fake-package + --package-db=relative/path fails

**To Reproduce**

```
$ cabal v2-repl -b some-package-in-db --package-db=relative/path/to/db
Resolving dependencies...
Build profile: -w ghc-9.0.2 -O1
In order, the following will be built (use -v for more details):
- fake-package-0 (lib) (first run)
Warning: this is a debug build of cabal-install with assertions enabled.
Configuring library for fake-package-0..
Error: cabal: The following package dependencies were requested
--dependency='some-package-in-db=some-package-in-db-0.1.0.0'
however the given installed package instance does not exist.
```

**Expected behavior**

Should not fail. I suspect the path gets interpreted relatively to the fake-package temporary directory. It should be made absolute before using it that way.

**System information**
- NixOS 22.05
- ghc 9.0.2, cabal 8.9-something

**Additional context**

cc @patrickdoc (#7676)

Related to #6977

Start by reproducing the `cabal v2-repl -b some-package-in-db --package-db=relative/path/to/db` command and tracing how fake-package creates its temporary directory and passes the package database path. Check the related context in #7676 and #6977. Done means the same relative package-db invocation resolves the installed dependency and no longer fails.

Written by the indexing model from the issue text.