xdg-desktop-portal: Use custom variable for finding portals

x-d-p only looks for portal definitions in one of two places:
- datadir (which we cannot install anything to, since Nix packages are immutable)
- when `XDG_DESKTOP_PORTAL_DIR` environment variable is set, the path specified therein
  (meant for tests, disables looking for portal configuration anywhere else)

Let’s introduce our own `NIX_XDG_DESKTOP_PORTAL_DIR` environment variable
that will only control the portal definitions lookup.
We will not use it for searching for configuration
because it would require looking in the parent directory
and `XDG_CONFIG_DIRS` variable is sufficient for us.

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