Fix sdist build for maturin itself

[messense]

cargo run sdist
   Compiling maturin v0.13.6 (/Users/messense/Projects/maturin)
    Finished dev [unoptimized + debuginfo] target(s) in 2.28s
     Running `target/debug/maturin sdist`
⚠️  Warning: `build-backend` in pyproject.toml is not set to `maturin`, packaging tools such as pip will not use maturin to build this project.
🍹 Building a mixed python/rust project
🔗 Found bin bindings
📡 Using build options bindings from pyproject.toml
💥 maturin failed
  Caused by: Failed to build source distribution
  Caused by: prefix not found

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[messense]

Will do a refactoring later to unify relative/absolute path handling related to strip_prefix.