use latest available genesis config for ALL migration tests

Use the genesis block from the latest release only.
Having different genesis block (hash) across releases will basically
make all wallets incompatible with each others. Prior to _now_ workers
would simply loop ad-infinitum trying to rollback to (0, 0).

Now, workers would simply exit complaining about a different genesis
hash. To work around this, we use the same genesis configuration for
all nodes in the migration tests, so that the hash remains the same
across all tests.

nix/migration-tests.nix

@@ -82,21 +82,26 @@ let
     # Create a script that runs the migration test against the server of
     # a certain release.
     testRelease = rel: let
-        walletPackages = importRelease rel;
+        targetRelease = importRelease rel;
+        # Use the genesis block from the latest release only.
+        # Having different genesis block (hash) across releases will basically
+        # make all wallets incompatible with each others. Prior to v2020-01-20,
+        # workers would simply loop ad-infinitum trying to rollback to (0, 0).
+        latestRelease = importRelease null;
       in pkgs.writeScript "launch-migration-test-${releaseName rel}.sh" ''
         #!${pkgs.runtimeShell}
 
         export PATH=${makeBinPath [
-          walletPackages.cardano-wallet-jormungandr
-          walletPackages.jormungandr
+          targetRelease.cardano-wallet-jormungandr
+          targetRelease.jormungandr
           migrationTest
           pkgs.bash
           pkgs.coreutils
         ]}
-        export src=${walletPackages.src}
+        export src=${latestRelease.src}
 
         exec ${./launch-migration-test.sh} "$@"
-      '' // { inherit (walletPackages) cardano-wallet-jormungandr; };
+      '' // { inherit (latestRelease) cardano-wallet-jormungandr; };
 
     # Create a test runner script for the given release.
     # The test scenario is quite simple at present.
@@ -180,6 +185,7 @@ let
         ''}
       '';
     };
+    latest = importRelease null;
   in
     # Create a directory with migration test scripts for each release version.
     # At the top level is a script that runs all tests.
@@ -190,7 +196,7 @@ let
       mkdir -p $out/${test.name}/data
       cp ${test.cardano-wallet-jormungandr}/bin/* $out/${test.name}
       cp ${test.runner} $out/${test.name}/${test.runner.name}
-      cp ${test.src}/lib/jormungandr/test/data/jormungandr/{block0.bin,config.yaml,secret.yaml} $out/${test.name}/data
+      cp ${latest.src}/lib/jormungandr/test/data/jormungandr/{block0.bin,config.yaml,secret.yaml} $out/${test.name}/data
 
       # append test to the run all script
       echo "${test.name}\${test.runner.name}" >> $out/runall.bat