nixos/tests/cagebreak: Fix the test
nixos/tests/cagebreak: Fix the test

Starting Cagebreak as X11 client doesn't work anymore as wlroots 0.13
started to require the DRI3 extension which isn't supported by LLVMpipe:
machine # [   13.508284] xsession[938]: 00:00:00.003 [ERROR] [backend/x11/backend.c:433] X11 does not support DRI3 extension
machine # [   13.666989] show_signal_msg: 62 callbacks suppressed
machine # [   13.666993] .cagebreak-wrap[938]: segfault at 8 ip 0000000000408574 sp 00007ffef76f2440 error 4 in .cagebreak-wrapped[407000+d000]
machine # [   13.670483] Code: f4 ff ff 4c 8b 84 24 70 01 00 00 8d 45 01 48 89 c5 49 8b 3c c0 48 85 ff 75 e4 4c 89 c7 e8 84 f4 ff ff 48 8b bc 24 18 01 00 00 <48> 8b 47 08 4c 8d 6f d8 48 8d 68 d8 48 39 df 75 0e eb 36 66 0f 1f
machine # [   13.518274] xsession[938]: 00:00:00.006 [ERROR] [../cagebreak.c:313] Unable to create the wlroots backend

The test broke after updating Cagebreak in #121652 (bf8679ba948).

XWayland still fails for unknown reasons:
Modifiers specified, but DRI is too old
libEGL warning: DRI2: failed to create dri screen
libEGL warning: NEEDS EXTENSION: falling back to kms_swrast
glamor: No eglstream capable devices found
glamor: 'wl_drm' not supported
Missing Wayland requirements for glamor GBM backend
Missing Wayland requirements for glamor EGLStream backend
Failed to initialize glamor, falling back to sw
00:00:03.534 [ERROR] [xwayland/server.c:252] waitpid for Xwayland fork
failed: No child processes
(EE) failed to write to XWayland fd: Broken pipe
/nix/store/kcm3x8695fgycf31grzl9fy5gggwpram-xterm-367/bin/xterm: Xt
error: Can't open display: :0

The fallback to software rendering is to be expected but it looks like
XWayland is crashing with "failed to write to XWayland fd: Broken pipe".

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