gnu: phodav: Use libsoup 2 instead of libsoup 3.

gnu: phodav: Use libsoup 2 instead of libsoup 3.

Looks like phodav requires libsoup 2. Example of failing build logs,
wanting libsoup 2.4: <https://ci.guix.gnu.org/build/584454/log/raw>

After swapping the dependency, tests were failing due to HOME not being
writable. I added a new build phase to account for this. I also removed
the start-virtual-dir-server phase which no longer seems required, the
tests pass without it.

* gnu/packages/gnome.scm (phodav): Fix build, tests.
[inputs]: Replace libsoup 3 with libsoup 2.
[arguments]<#:phases>[set-temporary-home]: New phase.
[arguments]<#:phases>[start-virtual-dir-server]: Remove phase.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>