build: fix BINDIR_DEPTH computation on Windows

[ret2libc]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the rizin book with the relevant information (if needed)

Detailed description

Before there was a special case for the root dir and it was implemented by using os.sep as the representation for the root. That did not work on Windows. This commit rewrite the BINDIR_DEPTH computation's logic to use Path objects instead.

Test plan

1. Download windows build
2. Install it under D:\ (or C:\ or whatever)
3. Run rizin -H and check that the various directories make sense.

Before the patch RZ_LIBDIR was like C:\bin\lib, but the correct value should be C:\bin.

Closing issues

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