fix segfault in cachix

patches/fix-nix-2.19.patch

@@ -0,0 +1,13 @@
+diff --git a/src/libutil/current-process.cc b/src/libutil/current-process.cc
+index 352a6a0..7435f58 100644
+--- a/src/libutil/current-process.cc
++++ b/src/libutil/current-process.cc
+@@ -35,6 +35,8 @@ unsigned int getMaxCPU()
+ 
+         auto cpuMax = readFile(cpuFile);
+         auto cpuMaxParts = tokenizeString<std::vector<std::string>>(cpuMax, " \n");
++        if (cpuMaxParts.size() != 2)
++        	return 0;
+         auto quota = cpuMaxParts[0];
+         auto period = cpuMaxParts[1];
+         if (quota != "max")

profiles/overlay.nix

@@ -38,6 +38,17 @@ with lib; {
         wine = prev.wineWow64Packages.stagingFull;
         intel-vaapi-driver = prev.intel-vaapi-driver.override { enableHybridCodec = true; };
 
+        haskellPackages = prev.haskellPackages.override {
+          overrides = _hFinal: hPrev: {
+            # TODO: remove after cachix starts to use nix > v2.21
+            cachix = let
+              nix-fixed = prev.nixVersions.nix_2_19.overrideAttrs (oldAttrs: {
+                patches = oldAttrs.patches ++ [ ../patches/fix-nix-2.19.patch ];
+              });
+            in hPrev.cachix.override { nix = nix-fixed; };
+          };
+        };
+
         modprobed-db = prev.modprobed-db.overrideAttrs (oa: {
           postPatch = (oa.postPatch or "") + ''
             substituteInPlace ./common/modprobed-db.in \