rustup self update doesn't update DisplayVersion

[ave9858]

Problem

When updating rustup from a prior release using rustup update, the DisplayVersion in the registry isn't updated from the old release version. This will result in tools like winget thinking rustup is outdated, and might confuse users.

Steps

1. Install an older release of rustup that sets DisplayVersion such as 1.26.0
2. Run rustup update, and check that rustup is updated to the latest version
3. Open the "Uninstall a program" menu in control panel or check HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\Rustup in the registry to see the outdated version number

Possible Solution(s)

Maybe do_add_to_programs isn't called for updates? I haven't checked the code yet.

Notes

Related to #3047 and #3055

Rustup version

rustup 1.27.0 (bbb9276d2 2024-03-08)
info: This is the version for the rustup toolchain manager, not the rustc compiler.
info: The currently active `rustc` version is `rustc 1.77.0 (aedd173a2 2024-03-17)`

Installed toolchains

Default host: x86_64-pc-windows-msvc
rustup home:  C:\Users\User\.rustup

stable-x86_64-pc-windows-msvc (default)
rustc 1.77.0 (aedd173a2 2024-03-17)