Add mkosi-as-caller helper

Add mkosi-as-caller helper

This helper can be used to replace the following pattern which is similar to
something used in the systemd codebase:

SU="setpriv --reuid=${MKOSI_UID:-0} --regid=${MKOSI_GID:-0} --clear-groups"
$SU meson setup "$BUILDDIR/build" "$SRCDIR"
$SU meson compile -C "$BUILDDIR/build"
meson install -C "$BUILDDIR/build" --no-rebuild

With the helper this becomes:

mkosi-as-caller meson setup "$BUILDDIR/build" "$SRCDIR"
mkosi-as-caller meson compile -C "$BUILDDIR/build"
meson install -C "$BUILDDIR/build" --no-rebuild