Harden the ci infrastructure for better security

Following some of the advice given in briansmith/untrusted#50.

.github/workflows/mobile_tests.yml

@@ -1,5 +1,7 @@
 name: Mobile Tests
 on: [push, pull_request]
+permissions:
+  contents: read
 
 jobs:
   mobile_tests:
@@ -11,6 +13,8 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
+      with:
+        persist-credentials: false
     - uses: WillAbides/[email protected]
       with:
         go-version: ${{ matrix.go-version }}

.github/workflows/platform_tests.yml

@@ -1,5 +1,7 @@
 name: Platform Tests
 on: [push, pull_request]
+permissions:
+  contents: read
 
 jobs:
   platform_tests:
@@ -12,6 +14,8 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
+      with:
+        persist-credentials: false
     - uses: WillAbides/[email protected]
       id: setup-go-faster
       with:

.github/workflows/static_analysis.yml

@@ -1,11 +1,15 @@
 name: Static Analysis
 on: [push, pull_request]
+permissions:
+  contents: read
 
 jobs:
   checks:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
+      with:
+        persist-credentials: false
     - uses: WillAbides/[email protected]
       with:
         go-version: '1.17.x'