ci: pin all actions to verified SHAs and clear zizmor findings

Pin every action to a commit SHA whose tag comment matches (verified via gh api),
add least-privilege permissions, set persist-credentials: false, and replace the
archived actions-rs/cargo with a plain cargo test. zizmor clean at default persona.

.github/workflows/install.yml

@@ -9,6 +9,9 @@ on:
 env:
   CARGO_TERM_COLOR: always
 
+permissions:
+  contents: read
+
 jobs:
   build:
     strategy:
@@ -19,7 +22,9 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6
+        with:
+          persist-credentials: false
       - name: Install termscp from script
         run: |
           ./install.sh -f