facilitate variant install when target package is read-only

[nerdvegas]

It's fairly common for a studio to run some post-process after a package release, which recursively sets dirs/files in the payload to read-only. This can make it difficult to mutate the package after the fact - eg, install a variant separately, or perform a package copy.

It would be helpful to add a configurable mode which temporarily (and if possible) removes read-only mode where necessary, during things like a variant install. This typically will only be possible if user is root, or same owner as the original package install.