Allow testing ssh:// and ssh-ng:// across versions too

We want to make sure both protocols don't break. Also recall ssh-ng://
will be slightly different than unix:// because the former transfers
files over the protocol too.

Add env vars for the remote (ssh/ssh-ng) store programs, we will use
them in the test suite soon.

(cherry picked from commit 4bf2dd5)

src/libstore/legacy-ssh-store.cc

@@ -16,7 +16,12 @@ struct LegacySSHStore : public Store
     const Setting<int> maxConnections{this, 1, "max-connections", "maximum number of concurrent SSH connections"};
     const Setting<Path> sshKey{this, "", "ssh-key", "path to an SSH private key"};
     const Setting<bool> compress{this, false, "compress", "whether to compress the connection"};
-    const Setting<Path> remoteProgram{this, "nix-store", "remote-program", "path to the nix-store executable on the remote system"};
+    const Setting<Path> remoteProgram{
+        this,
+        getEnv("_NIX_TEST_LEGACY_SSH_REMOTE_PROGRAM", "nix-store"),
+        "remote-program",
+        "path to the nix-store executable on the remote system",
+    };
     const Setting<std::string> remoteStore{this, "", "remote-store", "URI of the store on the remote system"};
 
     // Hack for getting remote build log output.

src/libstore/ssh-store.cc

@@ -16,8 +16,13 @@ class SSHStore : public RemoteStore
 
     const Setting<Path> sshKey{(Store*) this, "", "ssh-key", "path to an SSH private key"};
     const Setting<bool> compress{(Store*) this, false, "compress", "whether to compress the connection"};
-    const Setting<Path> remoteProgram{this, "nix-daemon", "remote-program", "path to the nix-daemon executable on the remote system"};
-    const Setting<std::string> remoteStore{this, "", "remote-store", "URI of the store on the remote system"};
+    const Setting<Path> remoteProgram{
+        (Store *) this,
+        getEnv("_NIX_TEST_SSH_REMOTE_PROGRAM", "nix-daemon"),
+        "remote-program",
+        "path to the nix-daemon executable on the remote system",
+    };
+    const Setting<std::string> remoteStore{(Store *) this, "", "remote-store", "URI of the store on the remote system"};
 
     SSHStore(const std::string & host, const Params & params)
         : Store(params)