Punt on improper global flags for now

See the note in the test.

We don't want these flags showing up for commands where they are
irrelevant.

Eventually, this needs a proper fix, but it need not be a blocker for
stabilize: for a quick-n-dirty punt, just put these flags behind the
`nix-command` unstable feature.

This is fine because they are only relevant for commands which we don't
need to stabilize for a while.

src/nix/main.cc

@@ -82,6 +82,7 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
             .description = "Print full build logs on standard error.",
             .category = loggingCategory,
             .handler = {[&]() { logger->setPrintBuildLogs(true); }},
+            .experimentalFeature = Xp::NixCommand,
         });
 
         addFlag({
@@ -97,6 +98,7 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
             .description = "Disable substituters and consider all previously downloaded files up-to-date.",
             .category = miscCategory,
             .handler = {[&]() { useNet = false; }},
+            .experimentalFeature = Xp::NixCommand,
         });
 
         addFlag({
@@ -104,6 +106,7 @@ struct NixArgs : virtual MultiCommand, virtual MixCommonArgs
             .description = "Consider all previously downloaded files out-of-date.",
             .category = miscCategory,
             .handler = {[&]() { refresh = true; }},
+            .experimentalFeature = Xp::NixCommand,
         });
     }