scripts/deploy: separate eval and build phases to decrease maximum ram requirements

scripts/deploy

@@ -154,10 +154,16 @@ deployOneHost() {
   local host="$1"
   local variant="$2"
 
-  info "building $host$variant ..."
-  destructive nix-build -A "hosts.$host$variant.toplevel" --out-link "./build/result-$host$variant" "${nixArgs[@]}" || return 1
-  storePath="$(readlink ./build/result-$host$variant)"
-  info "build $host$variant -> $storePath"
+  # `nix-build -A foo` evals and then realizes foo, but it never unloads the memory used to eval foo.
+  # my exprs are heavyweight, we need that memory for building, so do the evals separately from the realizations:
+  info "evaluating $host$variant..."
+  local drvPath=$(nix eval --raw -f . "hosts.$host$variant.toplevel.drvPath")
+  info "building $host$variant ($drvPath)"
+  local storePath=$(destructive nix-store --realize "$drvPath" "${nixArgs[@]}")
+  if [ -z "$storePath" ]; then
+    return 1
+  fi
+  info "built $host$variant -> $storePath"
 
   # mimic `nixos-rebuild --target-host`, in effect:
   # - nix-copy-closure ...