Mempool: use postPatch to copy lock file for npmDeps fetcher

prefetch-npm-deps runs in the deps fetcher's buildPhase and checks for
package-lock.json at CWD (repo root). postPatch runs in both the
fetcher and the main build, so we copy the lock file there. The main
build then cds into the subdir in buildPhase.

npmDepsHash values remain placeholders (lib.fakeHash).

packages/mempool/default.nix

@@ -80,11 +80,14 @@ in rec {
     pname = "mempool-backend";
     inherit version src;
 
-    # cd into the backend subdir during unpack so the npmDeps fetcher
-    # finds package-lock.json at its CWD. This hook runs in both the
-    # deps fetcher and the main build.
-    preBuild = ''
+    # postPatch runs in the npmDeps fetcher too. Copy the lock file to
+    # the repo root so prefetch-npm-deps can find it, then apply patches
+    # in the main build.
+    postPatch = ''
+      cp backend/package-lock.json .
       cd backend
+      patch -p1 < ${./0001-allow-disabling-mining-pool-fetching.patch}
+      cd ..
     '';
 
     # Placeholder: build once, then replace with the hash from the
@@ -96,22 +99,13 @@ in rec {
       rsync
     ];
 
-    # Apply patches only in the main build, not in the npmDeps fetcher.
-    # prePatch runs before patchPhase in the main build only.
-    prePatch = ''
-      cd backend
-    '';
-
-    postPatch = ''
-      patch -p1 < ${./0001-allow-disabling-mining-pool-fetching.patch}
-    '';
-
     dontNpmBuild = true;
     dontNpmInstall = true;
 
     buildPhase = ''
       runHook preBuild
 
+      cd backend
       patchShebangs node_modules
 
       ${sync} ${mempool-rust-gbt}/ rust-gbt
@@ -154,10 +148,10 @@ in rec {
     pname = "mempool-frontend";
     inherit version src;
 
-    # cd into the frontend subdir so the npmDeps fetcher finds
-    # package-lock.json at its CWD.
-    preBuild = ''
-      cd frontend
+    # postPatch runs in the npmDeps fetcher too. Copy the lock file to
+    # the repo root so prefetch-npm-deps can find it.
+    postPatch = ''
+      cp frontend/package-lock.json .
     '';
 
     # Placeholder: build once, then replace with the hash from the
@@ -175,6 +169,7 @@ in rec {
     buildPhase = ''
       runHook preBuild
 
+      cd frontend
       patchShebangs node_modules
 
       # sync-assets.js is called during `npm run build` and downloads assets from the