Revoke all permissions from GHA workflows where possible

.github/workflows/abi-cafe.yml

@@ -3,6 +3,8 @@ name: Abi-cafe
 on:
   - push
 
+permissions: {}
+
 jobs:
   abi_cafe:
     runs-on: ${{ matrix.os }}

.github/workflows/main.yml

@@ -8,6 +8,8 @@ defaults:
   run:
     shell: bash
 
+permissions: {}
+
 jobs:
   rustfmt:
     runs-on: ubuntu-latest

.github/workflows/rustc.yml

@@ -3,6 +3,8 @@ name: Various rustc tests
 on:
   - push
 
+permissions: {}
+
 jobs:
   bootstrap_rustc:
     runs-on: ubuntu-latest