Fix tests.remoteBuilds_local_nix_2_18

This was broken since a03bb44 because
Nix 2.18 does not support broken $SHELL settings. So don't try a
broken $SHELL on old Nix versions. (It's a mystery though why
tests.remoteBuilds_local_nix_2_13 and tests.remoteBuilds_local_nix_2_3
didn't fail...)

https://hydra.nixos.org/build/277366807

tests/nixos/remote-builds.nix

@@ -34,6 +34,8 @@ let
       }
     '';
 
+  supportsBadShell = lib.versionAtLeast config.nodes.client.nix.package.version "2.25pre";
+
 in
 
 {
@@ -82,7 +84,7 @@ in
             nix.settings.substituters = lib.mkForce [ ];
             programs.ssh.extraConfig = "ConnectTimeout 30";
             environment.systemPackages = [
-              # `bad-shell` is used to make sure Nix works an environment with a misbehaving shell.
+              # `bad-shell` is used to make sure Nix works in an environment with a misbehaving shell.
               #
               # More realistically, a bad shell would still run the command ("echo started")
               # but considering that our solution is to avoid this shell (set via $SHELL), we
@@ -125,13 +127,15 @@ in
             'echo hello world on $(hostname)' >&2
         """)
 
+      ${lib.optionalString supportsBadShell ''
       # Check that SSH uses SHELL for LocalCommand, as expected, and check that
       # our test setup here is working. The next test will use this bad SHELL.
       client.succeed(f"SHELL=$(which bad-shell) ssh -oLocalCommand='true' -oPermitLocalCommand=yes {builder1.name} 'echo hello world' | grep -F 'Hello, I am a broken shell'")
+      ''}
 
       # Perform a build and check that it was performed on the builder.
       out = client.succeed(
-        "SHELL=$(which bad-shell) nix-build ${expr nodes.client 1} 2> build-output",
+        "${lib.optionalString supportsBadShell "SHELL=$(which bad-shell)"} nix-build ${expr nodes.client 1} 2> build-output",
         "grep -q Hello build-output"
       )
       builder1.succeed(f"test -e {out}")