openssh: remove softhsm ref when cross-compiling

softhsm is used for tests, which don't run when cross-compiling, but it
was accidentally included anyway due to a mistake in the preCheck
conditionals. This fixes the cross build on armv6l.

pkgs/tools/networking/openssh/common.nix

···
   ++ lib.optional (!stdenv.hostPlatform.isDarwin) hostname
   ++ lib.optional (!stdenv.hostPlatform.isDarwin && !stdenv.hostPlatform.isMusl) softhsm;
 
-  preCheck =
-    lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+  preCheck = lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) (
+    ''
       # construct a dummy HOME
       export HOME=$(realpath ../dummy-home)
       mkdir -p ~/.ssh
···
       # The extra tests check PKCS#11 interactions, which softhsm emulates with software only
       substituteInPlace regress/test-exec.sh \
         --replace /usr/local/lib/softhsm/libsofthsm2.so ${lib.getLib softhsm}/lib/softhsm/libsofthsm2.so
-    '';
+    ''
+  );
   # integration tests hard to get working on darwin with its shaky
   # sandbox
   # t-exec tests fail on musl