nix run: Allow passing a command to execute

E.g.

  nix run nixpkgs.hello -c hello --greeting Hallo

Note that unlike "nix-shell --command", no quoting of arguments is
necessary.

"-c" (short for "--command") cannot be combined with "--" because they
both consume all remaining arguments. But since installables shouldn't
start with a dash, this is unlikely to cause problems.

src/libutil/args.cc

@@ -102,9 +102,11 @@ bool Args::processFlag(Strings::iterator & pos, Strings::iterator end)
         ++pos;
         Strings args;
         for (size_t n = 0 ; n < flag.arity; ++n) {
-            if (pos == end)
+            if (pos == end) {
+                if (flag.arity == ArityAny) break;
                 throw UsageError(format("flag '%1%' requires %2% argument(s)")
                     % name % flag.arity);
+            }
             args.push_back(*pos++);
         }
         flag.handler(args);

src/libutil/args.hh

@@ -26,6 +26,8 @@ public:
 
 protected:
 
+    static const size_t ArityAny = std::numeric_limits<size_t>::max();
+
     /* Flags. */
     struct Flag
     {

src/nix/run.cc

@@ -17,8 +17,20 @@ std::string chrootHelperName = "__run_in_chroot";
 
 struct CmdRun : InstallablesCommand
 {
+    Strings command = { "bash" };
+
     CmdRun()
     {
+        mkFlag()
+            .longName("command")
+            .shortName('c')
+            .description("command and arguments to be executed; defaults to 'bash'")
+            .arity(ArityAny)
+            .labels({"command", "args"})
+            .handler([&](Strings ss) {
+                if (ss.empty()) throw UsageError("--command requires at least one argument");
+                command = ss;
+            });
     }
 
     std::string name() override
@@ -43,8 +55,8 @@ struct CmdRun : InstallablesCommand
                 unixPath.push_front(path + "/bin");
         setenv("PATH", concatStringsSep(":", unixPath).c_str(), 1);
 
-        std::string cmd = "bash";
-        Strings args = { cmd };
+        std::string cmd = *command.begin();
+        Strings args = command;
 
         /* If this is a diverted store (i.e. its "logical" location
            (typically /nix/store) differs from its "physical" location