msi: use cargo-dist install-path

Currently msi installers default to installing to `%PROGRAMFILES%\$app_name\`, but we have an install-path system that the shell installers already use. While I think the *default* is good, it would be cool if we could make the two interop when desirable.

As far as I know this isn't exposed in cargo-wix, so:

1. find [some feature in WiXv3 that changes the default](https://wixtoolset.org/docs/v3/).
2. teach cargo-wix about it (add a config)
3. teach cargo-dist to set that (? in what cases)

Start by reading the WiX v3 documentation linked in the issue and comparing the existing install-path behavior in the shell installers. Define how cargo-wix should expose the setting and when cargo-dist should provide it; done requires MSI and shell installers to interoperate without changing the current default.

Written by the indexing model from the issue text.