Fix argument parsing for volta run to properly handle flags

Clap's parsing for variable-length argument lists has precedence rules for what
happens when a potential vararg overlaps with a known flag value (see https://docs.rs/clap/latest/clap/struct.Arg.html#method.allow_hyphen_values):

1. If we are already in a list of variable args, the args take precedence and any
   potential flags are ignored (flags are treated as part of the vararg)
2. If we are _not_ already in a list of variable args, then known flags take
   precedence and are parsed as such, rather than starting the vararg list

As a result of this precedence, with `volta run` set up to have `args` be
separate from `command`, we could run into parsing issues with commands that have
flags that overlap with Volta's. For example:

```
volta run node -h
```

When parsing this, Clap would see `node` as the `command`, and then it would
parse `-h` as _Volta's_ `-h` (help) flag and print Volta's help. This is because
the variable-length `args` hadn't started yet (we hadn't run into any unknown
positional arguments), so Clap treated `-h` as a known flag with higher
precedence. This is exactly the opposite of what we actually want for `volta run`

Instead, what we want is that everything after the `command` argument should
be treated as part of the arguments to the command; regardless of whether the
flag overlaps with Volta's flags.

To achieve that, we combine `command` and `args` into a _single_ vararg, so that
by virtue of having a command in the first place, the vararg has started parsing
and Clap will treat any flags as more argument values, rather than as flags.

src/command/run.rs

@@ -44,12 +44,14 @@ pub(crate) struct Run {
     #[arg(long = "env", value_name = "NAME=value", num_args = 1)]
     envs: Vec<String>,
 
-    /// The command to run
-    command: OsString,
-
-    /// Arguments to pass to the command
-    #[arg(allow_hyphen_values = true, trailing_var_arg = true)]
-    args: Vec<OsString>,
+    /// The command to run, along with any arguments
+    #[arg(
+        allow_hyphen_values = true,
+        trailing_var_arg = true,
+        value_name = "COMMAND",
+        required = true
+    )]
+    command_and_args: Vec<OsString>,
 }
 
 impl Command for Run {
@@ -59,7 +61,13 @@ impl Command for Run {
         let envs = self.parse_envs();
         let platform = self.parse_platform(session)?;
 
-        match execute_tool(&self.command, &self.args, &envs, platform, session).into_result() {
+        // Safety: At least one value is required for `command_and_args`, so there must be at
+        // least one value in the list. If no value is provided, Clap will show a "required
+        // argument missing" message and this function won't be called.
+        let command = &self.command_and_args[0];
+        let args = &self.command_and_args[1..];
+
+        match execute_tool(command, args, &envs, platform, session).into_result() {
             Ok(()) => {
                 session.add_event_end(ActivityKind::Run, ExitCode::Success);
                 Ok(ExitCode::Success)