[RaphaelFakhri]

Summary

convert_to_install_only rewrites each entry path from python/install/... to python/... with tar::Header::set_path. set_path writes the new path and a NUL terminator but leaves the rest of the 100-byte name field unchanged. It also leaves the ustar prefix field unchanged when the new path fits in name. Each rewritten header kept the tail of the original, longer path after the NUL.

The tar format allows this, but some readers include those bytes in the entry name. For example, older System.Formats.Tar releases in .NET read python/bin/idle3.13 as python/bin/idle3.13\0dle3.13 (dotnet/runtime#117331). In cpython-3.13.16+20261013-x86_64-unknown-linux-gnu-install_only.tar.gz, 4523 of 4526 headers have non-zero bytes after the terminator, for example python/bin/pip\0bin/pip.

This change clears name, and prefix for ustar headers, before calling set_path. The install_only_stripped archives are produced from the install_only archive without changing paths, so they are fixed too.

Fixes #686

Test plan

• Added release::tests::install_only_clears_stale_header_path_bytes. The test builds a small full archive, converts it, and checks the raw name and prefix bytes of every output header. It covers a short path (bin/idle3.13) and a path stored with a ustar prefix in the input that fits in name after conversion.
• Without the fix, the test fails on the name check. With only the name clearing, it fails on the prefix check. With the full fix, cargo test --bin pythonbuild passes (4 passed).
• cargo fmt and cargo clippy --bin pythonbuild --tests report nothing new.