chore(ci): harden workflow by setting default permission to read only (#320)

Defines per action permissions scoping, defaulting to read only

RE: 

```
      - uses: actions/checkout@v5
        with:
          persist-credentials: false
```

See: actions/checkout#485

This does not yet introduce pinning to hashes, I want to find a good way
to maintain this first. Apparently Dependabot has a feature for this.

Author: zerosnacks

.github/workflows/ci.yml

@@ -1,5 +1,8 @@
 name: CI
 
+permissions:
+  contents: read
+
 on:
   push:
     branches: [main]
@@ -30,6 +33,8 @@ jobs:
             flags: "--all-features"
     steps:
       - uses: actions/checkout@v5
+        with:
+          persist-credentials: false
       - uses: dtolnay/rust-toolchain@master
         with:
           toolchain: ${{ matrix.rust }}
@@ -50,6 +55,8 @@ jobs:
     timeout-minutes: 30
     steps:
       - uses: actions/checkout@v5
+        with:
+          persist-credentials: false
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
         with:
@@ -61,6 +68,8 @@ jobs:
     timeout-minutes: 30
     steps:
       - uses: actions/checkout@v5
+        with:
+          persist-credentials: false
       - uses: dtolnay/rust-toolchain@stable
       - uses: taiki-e/install-action@cargo-hack
       - uses: Swatinem/rust-cache@v2
@@ -74,9 +83,9 @@ jobs:
     timeout-minutes: 30
     steps:
       - uses: actions/checkout@v5
-      - uses: dtolnay/rust-toolchain@stable
         with:
-          components: clippy
+          persist-credentials: false
+      - uses: dtolnay/rust-toolchain@clippy
       - uses: Swatinem/rust-cache@v2
         with:
           cache-on-failure: true
@@ -89,6 +98,8 @@ jobs:
     timeout-minutes: 30
     steps:
       - uses: actions/checkout@v5
+        with:
+          persist-credentials: false
       - uses: dtolnay/rust-toolchain@nightly
       - uses: Swatinem/rust-cache@v2
         with:
@@ -102,6 +113,8 @@ jobs:
     timeout-minutes: 30
     steps:
       - uses: actions/checkout@v5
+        with:
+          persist-credentials: false
       - uses: dtolnay/rust-toolchain@nightly
         with:
           components: rustfmt
@@ -113,6 +126,7 @@ jobs:
   ci-success:
     runs-on: ubuntu-latest
     if: always()
+    permissions: {}
     needs:
       - test
       - doctest