[nix] separate build phase for mlirbc builder

This commit simplify the process developers should take for debugging
elaborator.

To get parameter that feeds to elaborator, developer can now run the
following command line:

```bash
nix develop .#t1.<config-name>.<top-name>.mlirbc
eval "$configGenPhase"
ls stage1
```

Signed-off-by: Avimitin <[email protected]>

nix/t1/conversion/chisel-to-mlirbc.nix

@@ -22,23 +22,30 @@ stdenvNoCC.mkDerivation {
 
   nativeBuildInputs = [ espresso circt ];
 
-  buildCommand = ''
-    mkdir -p "$out"
-
+  configGenPhase = ''
     mkdir stage1 && pushd stage1
 
-    elaborateProc="${elaborator}/bin/elaborator ${generatorClassName} ${elaboratorArgs}"
-    echo "[nix] Generating config with cmd opt: $elaborateProc"
-    eval "$elaborateProc"
+    ${elaborator}/bin/elaborator ${generatorClassName} ${elaboratorArgs}
+
     popd
+  '';
 
+  elaboratePhase = ''
     mkdir stage2 && pushd stage2
+
     echo "[nix] Elaborating with generated config"
     ${elaborator}/bin/elaborator ${generatorClassName} design --parameter ../stage1/*.json
+  '';
+
+  buildCommand = ''
+    mkdir -p "$out"
+
+    echo "[nix] Generating config with cmd opt: $configGenPhase"
+    eval "$configGenPhase"
+
+    echo "[nix] Elaborate mlirbc with cmd opt: $elaboratePhase"
+    eval "$elaboratePhase"
 
-    # ---------------------------------------------------------------------------------
-    # Run circt toolchain
-    # ---------------------------------------------------------------------------------
     echo "[nix] elaborate finish, parsing output with firtool"
     firtool *.fir \
       --annotation-file *.json \