ci: use minimal permissions for Github Actions (#351)

Fixes #348

we only need to read the contents of the repo to run our tests, no other
permissions are needed, as we currently do not publish via our CI jobs.

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

.github/workflows/tests.yml

@@ -8,6 +8,9 @@ on:
   schedule:
     - cron: "0 12 * * 1"
 
+permissions:
+  contents: read
+
 env:
   CARGO_INCREMENTAL: 0
   RUSTFLAGS: "-Dwarnings"