don't run GHA workflows on tag pushes (#6510)

Per the branching experiment review we are about to begin in the control
plane huddle.

1. For the rust.yml workflow, only run the `push` mode when pushing to
`main` or any branch matching the glob `rel/**`.
2. For the hakari.yml workflow, apply the same rules. This has the
effect of enabling them for the release branches, and also enabling them
for pull requests that do not target the `main` branch.

.github/workflows/hakari.yml

@@ -4,9 +4,8 @@ on:
   push:
     branches:
       - main
-  pull_request:
-    branches:
-      - main
+      - 'rel/**'
+  pull_request: {}
 
 name: cargo hakari
 

.github/workflows/rust.yml

@@ -3,7 +3,12 @@
 #
 name: Rust
 
-on: [ push, pull_request ]
+on:
+  push:
+    branches:
+      - main
+      - 'rel/**'
+  pull_request: {}
 
 jobs:
   check-style: