chore: Set permissions for GitHub actions

 Restrict the GitHub token permissions only to the required ones; this way, even if the attackers will succeed in compromising your workflow, they won’t be able to do much.

- Included permissions for the action. https://github.com/ossf/scorecard/blob/main/docs/checks.md#token-permissions

https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#permissions

https://docs.github.com/en/actions/using-jobs/assigning-permissions-to-jobs

[Keeping your GitHub Actions and workflows secure Part 1: Preventing pwn requests](https://securitylab.github.com/research/github-actions-preventing-pwn-requests/)

Signed-off-by: nathannaveen <42319948+nathannaveen@users.noreply.github.com>

.github/workflows/doc.yml

@@ -2,6 +2,9 @@ name: doc
 
 on: [push, pull_request]
 
+permissions:
+  contents: read
+
 jobs:
   build:
     # Use Ubuntu 20.04 because doxygen 1.8.13 from Ubuntu 18.04 is broken.

.github/workflows/linux.yml

@@ -2,6 +2,9 @@ name: linux
 
 on: [push, pull_request]
 
+permissions:
+  contents: read
+
 jobs:
   build:
     runs-on: ${{ matrix.os }}

.github/workflows/macos.yml

@@ -2,6 +2,9 @@ name: macos
 
 on: [push, pull_request]
 
+permissions:
+  contents: read
+
 jobs:
   build:
     runs-on: macos-10.15

.github/workflows/windows.yml

@@ -2,6 +2,9 @@ name: windows
 
 on: [push, pull_request]
 
+permissions:
+  contents: read
+
 jobs:
   build:
     runs-on: ${{matrix.os}}