"Make targetDir relative to current directory" often doesn't work

In #7581, a call to `makeRelativeToCurrentDirectory` was added in an attempt to shorten command lines. However, in practice, [this call](https://github.com/haskell/cabal/blob/3c06125115c2a86324e9f72f615f381278bb107b/Cabal/src/Distribution/Simple/GHC/Build.hs#L100) to `makeRelative` will fail to make the path relative. For example, with a directory structure like

```
cabal.project
p/p.cabal
```

when building `p` the arguments will look like:

```
buildTargetDir_absolute = /long/working/directory/build/dist-newstyle/more/stuff/
currentWorkingDirectory = /long/working/directory/p/
```

Note the `/p/` at the end of the current working directory, which means that we fail to make the build target directory relative to the current working directory.

The tests `keep-going`, `regression/3324` and `local-tarball` in the `integration-tests2` testsuite all fail if one adds an error after that invocation of `makeRelativeToCurrentDirectory` when it doesn't make the path relative.

Start in Cabal/src/Distribution/Simple/GHC/Build.hs at the makeRelativeToCurrentDirectory call, using the p/p.cabal layout and paths from the report to reproduce the failure. Then run the integration-tests2 tests keep-going, regression/3324, and local-tarball; done means the target directory is made relative when expected and these tests no longer expose the failure.

Written by the indexing model from the issue text.