fix(ops/pipelines): explicitly set contexts for annotations

I think what might be going on with b/231 is that the annotations
somehow started conflicting because they don't have contexts set.

Lets try setting a context and see if it changs anything ...

Change-Id: I62ed57f9e24f08e4e7215f05d35cfa769e2e2c24
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7640
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tools/rust-crates-advisory/default.nix

@@ -114,6 +114,7 @@ depot.nix.readTree.drvTargets {
       command = buildkiteReportStep {
         command = [ depot-rust-crates-advisory-report ];
         style = "warning";
+        context = "depot-crate-advisories";
       };
     };
   };