Use multiple subdirs in cabal.project.

Alas, it seems like you can't use '.' as a subdir for some reason, so
there are still two entries for `cardano-prelude`.

cabal.project

@@ -25,28 +25,11 @@ source-repository-package
   location: https://github.com/input-output-hk/cardano-base
   tag: 7d795c3040ea7785812efa1c97864bbb41b15d3e
   --sha256: 130i0yj4y9br1m2bhisi6wni3f40i31nfhg878hv0kwi17chl9sz
-  subdir: binary
-
-source-repository-package
-  type: git
-  location: https://github.com/input-output-hk/cardano-base
-  tag: 7d795c3040ea7785812efa1c97864bbb41b15d3e
-  --sha256: 130i0yj4y9br1m2bhisi6wni3f40i31nfhg878hv0kwi17chl9sz
-  subdir: binary/test
-
-source-repository-package
-  type: git
-  location: https://github.com/input-output-hk/cardano-base
-  tag: 7d795c3040ea7785812efa1c97864bbb41b15d3e
-  --sha256: 130i0yj4y9br1m2bhisi6wni3f40i31nfhg878hv0kwi17chl9sz
-  subdir: cardano-crypto-class
-
-source-repository-package
-  type: git
-  location: https://github.com/input-output-hk/cardano-base
-  tag: 7d795c3040ea7785812efa1c97864bbb41b15d3e
-  --sha256: 130i0yj4y9br1m2bhisi6wni3f40i31nfhg878hv0kwi17chl9sz
-  subdir: slotting
+  subdir:
+    binary
+    binary/test
+    cardano-crypto-class
+    slotting
 
 source-repository-package
   type: git
@@ -65,7 +48,8 @@ source-repository-package
   location: https://github.com/input-output-hk/cardano-prelude
   tag: 316c854d1d3089f480708ad5cd5ecf8a74423ddd
   --sha256: 1srbl3jrkmpwypdz0yrx4nmah3qcsr93dp48zx2bamg51c4hcsyj
-  subdir: test
+  subdir:
+    test
 
 source-repository-package
   type: git

shell.nix

@@ -20,14 +20,17 @@ let
 
     # These programs will be available inside the nix-shell.
     buildInputs = with haskellPackages; [
-      cabal-install
       niv
       pkg-config
       hlint
       weeder
       ormolu.ormolu
     ];
 
+    tools = {
+      cabal = "3.2.0.0";
+    };
+
     # Prevents cabal from choosing alternate plans, so that
     # *all* dependencies are provided by Nix.
     exactDeps = false;