Configure permissions of GITHUB_TOKEN in workflow

`GITHUB_TOKEN` is an access token that is automatically generated and made accessible for use in GitHub Actions workflow
runs. The global default permissions of this token for workflow runs in a trusted context (i.e., not triggered by a
`pull_request` event from a fork) are set in the GiHub enterprise/organization/repository's administrative settings,
giving it either read-only or write permissions in all scopes.

In the case of a read-only default configuration, any workflow operations that require write permissions would fail with
an error like:

> 403: Resource not accessible by integration

In the case of a write default configuration, workflows have unnecessary permissions, which violates the security
principle of least privilege.

For this reason, GitHub Actions now allows fine grained control at a per-workflow or per-workflow job scope of the
permissions provided to the token. This is done using the `permissions` workflow key, which is used here to configure
the workflows for only the permissions require by each individual job.

I chose to always configure permissions at the job level even though in some cases the same permissions configuration
could be used for all jobs in a workflow. Even if functionally equivalent, I think it is semantically more appropriate
to always set the permissions at the job scope since the intention is to make the most granular possible permissions
configuration. Hopefully this approach will increase the likelihood that appropriate permissions configurations will be
made in any additional jobs that are added to the workflows in the future.

The automatic permissions downgrade from write to read for workflow runs in an untrusted context (e.g., triggered by a
`pull_request` event from a fork) is unaffected by this change.

Even when all permissions are withheld (`permissions: {}`), the token still provides the authenticated API request rate
limiting allowance (authenticating API requests to avoid rate limiting is a one of the uses of the token in these
workflows).

Read permissions are required in the "contents" scope in order to checkout private repositories. Even though those
permissions are not required when the workflows are installed in this public repository, the templates are intended to
be applicable in public and private repositories both and so a small excess in permissions was chosen in order to use
the upstream templates unmodified.

Author: per1234

.github/workflows/sync-labels.yml

@@ -24,6 +24,8 @@ env:
 jobs:
   check:
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
 
     steps:
       - name: Checkout repository
@@ -55,6 +57,7 @@ jobs:
   download:
     needs: check
     runs-on: ubuntu-latest
+    permissions: {}
 
     strategy:
       matrix:
@@ -79,6 +82,9 @@ jobs:
   sync:
     needs: download
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      issues: write
 
     steps:
       - name: Set environment variables