Fix handling of kde_projects.xml on first-run.

Specifically, on first run in --pretend mode, where kdesrc-build has no
existing metadata of any sort downloaded, and no way to make a source
dir to hold incoming metadata.

Instead we just save it in a temp file.

The kde-build-metadata module still doesn't get downloaded though, and
so if you (for whatever reason) run kdesrc-build --pretend as your first
action upon installing kdesrc-build, you would not get 100% accurate
build plans.

Instead of special-casing that I think I would just add a --init or
--setup command line option and then tell the user to use that for
"--pretend in first-use" scenarios.

BUG:295715