diff --git a/.github/workflows/nix-portable.yml b/.github/workflows/nix-portable.yml
index fa9cbd3..f4c3f7a 100644
--- a/.github/workflows/nix-portable.yml
+++ b/.github/workflows/nix-portable.yml
@@ -3,7 +3,7 @@ name: "build and test"
 on:
   pull_request:
   push:
-    branches: [ "*" ]
+    branches: [ "ci", "master" ]
 jobs:
 
 
diff --git a/flake.lock b/flake.lock
index 511f8bb..367017a 100644
--- a/flake.lock
+++ b/flake.lock
@@ -1,20 +1,5 @@
 {
   "nodes": {
-    "defaultChannel": {
-      "locked": {
-        "lastModified": 1635350005,
-        "narHash": "sha256-tAMJnUwfaDEB2aa31jGcu7R7bzGELM9noc91L2PbVjg=",
-        "owner": "NixOS",
-        "repo": "nixpkgs",
-        "rev": "1c1f5649bb9c1b0d98637c8c365228f57126f361",
-        "type": "github"
-      },
-      "original": {
-        "id": "nixpkgs",
-        "ref": "nixos-20.09",
-        "type": "indirect"
-      }
-    },
     "lowdown-src": {
       "flake": false,
       "locked": {
@@ -69,7 +54,6 @@
     },
     "root": {
       "inputs": {
-        "defaultChannel": "defaultChannel",
         "nix": "nix",
         "nixpkgs": "nixpkgs"
       }
diff --git a/flake.nix b/flake.nix
index 3de884a..f8ae6d5 100644
--- a/flake.nix
+++ b/flake.nix
@@ -3,11 +3,6 @@
 
     nixpkgs.url = "nixpkgs/nixos-21.11";
 
-    # the nixpkgs version shipped with the nix-portable executable
-    # TODO: find out why updating this leads to error when building pkgs.hello:
-    # Error: checking whether build environment is sane... ls: cannot access './configure': No such file or directory
-    defaultChannel.url = "nixpkgs/nixos-20.09";
-
     nix.url = "nix/2.5.1";
     nix.inputs.nixpkgs.follows = "nixpkgs";
   };
@@ -103,7 +98,6 @@
 
       nixPortableForSystem = { system, crossSystem ? null,  }:
         let
-          pkgsDefaultChannel = import inp.defaultChannel { inherit system crossSystem; };
           pkgs = import inp.nixpkgs { inherit system crossSystem; };
           pkgsCached = if crossSystem == null then pkgs else import inp.nixpkgs { system = crossSystem; };
 
@@ -114,12 +108,10 @@
           # crashes if nixpkgs updated: error: executing 'git': No such file or directory
           pkgs.callPackage ./default.nix {
 
-            inherit proot;
-
-            pkgs = pkgsDefaultChannel;
+            inherit proot pkgs;
 
             lib = inp.nixpkgs.lib;
-            compression = "zstd -3 -T1";
+            compression = "zstd -18 -T0";
 
             nix = inp.nix.packages."${system}".nix;