Support unprivileged `.pkg` installation on MacOS

[multimeric]

The regular .pkg installer seems to require admin. It should be possible to create an installation configuration that's "my user only" that doesn't require sudo, e.g. this is what Quarto now does.

e.g. this appears when I try to install https://github.com/r-lib/rig/releases/download/v0.7.1/rig-0.7.1-macOS-arm64.pkg:

[gaborcsardi]

With https://github.com/portable-r/portable-r-macos there is hope that we can support this, but it'll be a long time before rig could use that instead of the CRAN build. A lot of current tools assume that R is installed in /Library/Frameworks.