mock_bin_src.rs: Localise import of std::fs::File
mock_bin_src.rs: Localise import of std::fs::File

The import of std::fs::File was only used in the windows
codepath, so localise its use to remove unused import warnings
otherwise issued in the tests.

Signed-off-by: Daniel Silverstone <3cc17a76d780e0cd3d87bad586a1c53967be5625@digital-scurf.org>

nodakai/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,rust-lang/rustup,rust-lang/rustup,rust-lang/rustup,inejge/rustup.rs,inejge/rustup.rs,nodakai/rustup.rs,inejge/rustup.rs,inejge/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,inejge/rustup.rs