CI: reduce ssh-key cargo hack permutations (#224)

Reduces the combinatorial explosion by isolating certain features and
testing them separately

.github/workflows/ssh-key.yml

@@ -87,7 +87,7 @@ jobs:
         with:
           toolchain: ${{ matrix.rust }}
       - uses: RustCrypto/actions/cargo-hack-install@master
-      - run: cargo hack test --feature-powerset --exclude-features crypto,default,encryption,getrandom,std --release
+      - run: cargo hack test --feature-powerset --exclude-features crypto,default,dsa,getrandom,p256,p384,p521,rsa,tdes,std --release
       - run: cargo test --release
       - run: cargo test --release --features crypto
       - run: cargo test --release --features encryption