Add write permissions for this GitHub token

This is already configured in the settings for this repo; adding it to
the workflow file ensures I'll have it if I use this as a template for
future projects.

.github/workflows/upload_binaries.yml

@@ -36,6 +36,8 @@ jobs:
             os: windows-latest
 
     runs-on: ${{ matrix.os || 'ubuntu-latest' }}
+    permissions:
+      contents: write
     steps:
       - uses: actions/checkout@v4
       - uses: taiki-e/upload-rust-binary-action@v1