openssh: disable t-exec tests on musl

These tests fail on musl, as encountered by Alpine:
https://github.com/alpinelinux/aports/blob/04d4e07ad8bd54fe49fe32c117c47d91f31ae937/main/openssh/APKBUILD#L132

We can exclude this test category for now, similarly to darwin.

pkgs/tools/networking/openssh/common.nix

@@ -151,7 +151,8 @@ stdenv.mkDerivation rec {
   '';
   # integration tests hard to get working on darwin with its shaky
   # sandbox
-  checkTarget = optional (!stdenv.isDarwin) "t-exec"
+  # t-exec tests fail on musl
+  checkTarget = optional (!stdenv.isDarwin && !stdenv.hostPlatform.isMusl) "t-exec"
     # other tests are less demanding of the environment
     ++ [ "unit" "file-tests" "interop-tests" ];