chore(tvix): move buildkite.yml from views/tvix to tvix/

This should remove the ambiguity on where buildkite.yml should show up
at least. We might still want to move workspace.josh into tvix/ too at
some point, but that's blocked on
https://github.com/josh-project/josh/issues/1287

This commit doesn't cause any change in views/tvix, with a previous
version of this commit added to my HEAD, I ran josh-filter to verify it
still points to ce0df88561fee8a4d6052ca9f2a1792f899a202a.

Change-Id: Ic03e92260dc7725bc2fcf01b7c72a097d92c028e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9828
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>

tvix/buildkite.yml

@@ -0,0 +1,9 @@
+# Build pipeline for the filtered //views/tvix workspace of depot. This
+# pipeline is triggered by each build of canon.
+#
+# Pipeline status is visible on https://buildkite.com/tvl/tvix
+
+steps:
+  - label: ":crab: cargo build"
+    command: |
+      nix-shell --run "cargo build && cargo test"