`.exe` could be added to custom builds if the extension is missing

Testing out cargo dist on a basic Go project here: https://github.com/alexanderjophus/hello-world-go/tree/005e332f775dd79568c3a94ae192045b786741f2

Note: I haven't developed anything for windows before, so not sure how I should handle .exe, however I _think_ that cargo dist should handle renaming the binary from `my_binary` to `my_binary.exe`.

This was the error from that run: https://github.com/alexanderjophus/hello-world-go/actions/runs/10433209110/job/28894711643

Reproduce the issue with the linked hello-world-go project and its GitHub Actions run, then trace cargo-dist's custom-build handling for Windows binaries. Done means a custom Windows binary without an extension is given the .exe extension and the linked build completes successfully.

Written by the indexing model from the issue text.