nickel: switch optional nix-experimental input to nix_2_31

The nix-experimental feature compiles against the Nix 2.31 headers, but
libnixmain now tries to create /nix/var/nix/profiles during
initialisation, which the build sandbox rejects. Skip the affected
integration test when enableNixImport is set.

The default build (enableNixImport = false) is bit-identical.

pkgs/by-name/ni/nickel/package.nix

@@ -41,7 +41,7 @@ rustPlatform.buildRustPackage (finalAttrs: {
   ];
 
   buildInputs = lib.optionals enableNixImport [
-    nixVersions.nix_2_28
+    nixVersions.nix_2_31
     boost
   ];
 
@@ -70,6 +70,12 @@ rustPlatform.buildRustPackage (finalAttrs: {
     "--exclude=py-nickel"
   ];
 
+  checkFlags = lib.optionals enableNixImport [
+    # libnixmain from Nix >= 2.31 tries to create /nix/var/nix/profiles on
+    # initialisation, which is rejected by the build sandbox.
+    "--skip=stdin_format::evaluates_nix_from_stdin"
+  ];
+
   postInstall = ''
     mkdir -p $nls/bin
     mv $out/bin/nls $nls/bin/nls