run: Don't let XDG_RUNTIME_DIR from user override the value we set

We use `bwrap --setenv XDG_RUNTIME_DIR` to set it to `/run/user/UID`,
regardless of what it is on the host system, but the changes made
to resolve CVE-2021-21261 unintentionally broke this by overwriting it
with the user's XDG_RUNTIME_DIR.

In practice this worked for most people, who either have
XDG_RUNTIME_DIR set to the same value we use (which is the conventional
setup from systemd-logind and elogind), or entirely unset (if they do not
have systemd-logind or elogind). However, it broke Wayland and other
XDG_RUNTIME_DIR-based protocols for people who intentionally set up an
XDG_RUNTIME_DIR that is different.

Fixes: 6d1773d2 "run: Convert all environment variables into bwrap arguments"
Resolves: https://github.com/flatpak/flatpak/issues/4372


Signed-off-by: Simon McVittie <smcv@collabora.com>