chore: defer file closes

[Haroka-74]

What?

Replaced manual Close() calls on every error path in the binary copy block of runUpdateCLI with defer, and report close errors on out via the named return value.

Why?

The previous manual Close() calls on every return path were fragile and silently discarded flush errors on out.

Closes #1056

[Haroka-74]

@andrinoff

I need to change this if err := os.Rename(tmpNew, execPath); err != nil { to if err = os.Rename(tmpNew, execPath); err != nil {.

Am I right?