revert default providers to 'wheel,sdist,nixpkgs'

mach_nix/VERSION

@@ -0,0 +1 @@
+conda-beta

mach_nix/nix/conda-channels.nix

@@ -75,6 +75,10 @@ let
 in
 if missingChannels != [] then
   throw "Conda channels [${toString missingChannels}] are unknown. Use 'condaChannelsExtra' to make them available"
-else
-  trace "using conda channels: ${toString (concatStringsSep ", " (attrNames allCondaChannels))}"
-  { inherit condaChannelsJson; }
+else let
+  channelNames = (attrNames allCondaChannels); in
+  if channelNames != [] then
+    trace "using conda channels: ${toString (concatStringsSep ", " (attrNames allCondaChannels))}"
+    { inherit condaChannelsJson; }
+  else
+    { inherit condaChannelsJson; }

mach_nix/provider_defaults.toml

@@ -1,4 +1,4 @@
-_default =                "conda,wheel,sdist,nixpkgs"
+_default =                "wheel,sdist,nixpkgs"
 
 gdal =                    "nixpkgs"
 pip =                     "nixpkgs,sdist"

tests/all-tests.nix

@@ -6,9 +6,14 @@ let
       import file ({
         inherit mach-nix;
       } // (if conda then (rec {
+        baseArgsMkPython = { _provierDefaults = (fromTOML (readFile ./mach_nix/provider_defaults.toml) // {
+          _default = "conda,wheel,sdist,nixpkgs";
+        }); };
+        baseArgsBuildPythonPackage = baseArgsMkPython;
+      }) else {
         baseArgsMkPython = { _provierDefaults = fromTOML (readFile ./mach_nix/provider_defaults.toml); };
         baseArgsBuildPythonPackage = baseArgsMkPython;
-      }) else {}));
+      }));
 in
 flatten (map (file: makeTests) [
   ./test_alias_dateutil.nix