give minimum permissions to existing GHA workflows (#70)

diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index c541752..2eb9cc9 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -8,6 +8,9 @@
     branches:
       - main
 
+permissions:
+  contents: read
+
 env:
   GOPROXY: https://proxy.golang.org/