argon2: test using cargo careful (#553)

Helps spot issues with `unsafe` code. See:

https://github.com/RalfJung/cargo-careful

.github/workflows/argon2.yml

@@ -3,6 +3,7 @@ name: argon2
 on:
   pull_request:
     paths:
+      - ".github/workflows/argon2.yml"
       - "argon2/**"
       - "Cargo.*"
   push:
@@ -42,7 +43,7 @@ jobs:
   minimal-versions:
     uses: RustCrypto/actions/.github/workflows/minimal-versions.yml@master
     with:
-        working-directory: ${{ github.workflow }}
+      working-directory: ${{ github.workflow }}
 
   test:
     runs-on: ubuntu-latest
@@ -75,6 +76,14 @@ jobs:
       - run: cargo test
       - run: cargo test --all-features
 
+  careful:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@nightly
+      - run: cargo install cargo-careful
+      - run: cargo careful test --all-features
+
   cross:
     strategy:
       matrix: