Use minimal GITHUB_TOKEN permissions for metrics and subtree workflow (#552)

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This PR uses minimal GitHub Token permissions for writing to a branch
and creating a PR instead of relying on overly permissive defaults.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

Author: AdnaneKhan

.github/workflows/kani-metrics.yml

@@ -11,6 +11,9 @@ defaults:
 
 jobs:
   update-kani-metrics:
+    permissions:
+      contents: write
+      pull-requests: write
     if: github.repository == 'model-checking/verify-rust-std'
     runs-on: ubuntu-latest
 

.github/workflows/update-subtree.yml

@@ -11,6 +11,9 @@ defaults:
 
 jobs:
   update-subtree-library:
+    permissions:
+      contents: write
+      pull-requests: write
     if: github.repository == 'model-checking/verify-rust-std'
     # Changing the host platform may alter the libgit2 version as used by
     # splitsh-lite, which will require changing the version of git2go.