refactor(tvix): move logo to //tvix:logo

Also expose both formats, then use it from
users/tazjin/presentations/tvix-eval-2023.

Change-Id: Id906e8aff5510a7a4f33336326472e86db18ea32
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9280
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/website/default.nix

@@ -28,5 +28,5 @@ in
 pkgs.runCommand "tvix-website" { } ''
   mkdir $out
   cp ${landing} $out/index.html
-  cp ${./tvix-logo.webp} $out/tvix-logo.webp
+  cp ${depot.tvix.logo}/logo.webp $out/
 ''

tvix/website/landing-en.md

@@ -1,4 +1,4 @@
-<img class="tvl-logo" src="./tvix-logo.webp"
+<img class="tvl-logo" src="./logo.webp"
      alt="A candy bar in different shades of blue that says 'Tvix by TVL' on it">
 
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