Improve and Debug CI compile-check-no-std Command (#15935)

# Objective

- Fix bug where `cargo run -p ci` fails due to differing implementations
for default values between `Default` trait and `argh`
- Automatically install target via `rustup` to make first-run simpler.

## Solution

The command will now attempt to install the target via `rustup` by
default, which will provide a cleaner error message if a malformed
target is passed. It will also avoid confusion when people attempt to
use this tool for the first time without the target already installed.

I've added a flag, --skip-install, to disable the attempted installation
just-in-case. (e.g., maybe rustup isn't in their path but cargo is?).

Also fixed a bug where the default value for `target` was different
between the `Default` trait and `argh`, causing `cargo run -p ci` to
fail.

## Testing

- CI
- Subcommand ran directly

## Notes

This issue was originally discovered by @targrub (on Discord):

> Unfortunately, running `cargo run -p ci` still gives that same error
as I initially reported (though `cargo run -p ci --
compile-check-no-std` succeeds). This is after having run `rustup target
add x86_64-unknown-none`.

tools/ci/src/commands/compile_check_no_std.rs

@@ -3,39 +3,65 @@ use argh::FromArgs;
 use xshell::cmd;
 
 /// Checks that the project compiles for a `no_std` target.
-#[derive(FromArgs, Default)]
+/// Note that this tool will attempt to install the target via rustup.
+/// This can be skipped by passing the "--skip-install" flag.
+#[derive(FromArgs)]
 #[argh(subcommand, name = "compile-check-no-std")]
 pub struct CompileCheckNoStdCommand {
     /// the target to check against.
     /// Defaults to "x86_64-unknown-none"
-    #[argh(option, default = "String::from(\"x86_64-unknown-none\")")]
+    #[argh(option, default = "Self::default().target")]
     target: String,
+    /// skip attempting the installation of the target.
+    #[argh(switch)]
+    skip_install: bool,
+}
+
+impl Default for CompileCheckNoStdCommand {
+    fn default() -> Self {
+        Self {
+            target: String::from("x86_64-unknown-none"),
+            skip_install: false,
+        }
+    }
 }
 
 impl Prepare for CompileCheckNoStdCommand {
     fn prepare<'a>(&self, sh: &'a xshell::Shell, _flags: Flag) -> Vec<PreparedCommand<'a>> {
         let target = self.target.as_str();
-        vec![PreparedCommand::new::<Self>(
+        let mut commands = Vec::new();
+
+        if !self.skip_install {
+            commands.push(PreparedCommand::new::<Self>(
+                cmd!(sh, "rustup target add {target}"),
+                "Unable to add the required target via rustup, is it spelled correctly?",
+            ));
+        }
+
+        commands.push(PreparedCommand::new::<Self>(
             cmd!(
                 sh,
                 "cargo check -p bevy_ptr --no-default-features --target {target}"
             ),
             "Please fix compiler errors in output above for bevy_ptr no_std compatibility.",
-        ),
-        PreparedCommand::new::<Self>(
+        ));
+
+        commands.push(PreparedCommand::new::<Self>(
             cmd!(
                 sh,
                 "cargo check -p bevy_utils --no-default-features --target {target}"
             ),
             "Please fix compiler errors in output above for bevy_utils no_std compatibility.",
-        ),
-        PreparedCommand::new::<Self>(
+        ));
+
+        commands.push(PreparedCommand::new::<Self>(
             cmd!(
                 sh,
                 "cargo check -p bevy_mikktspace --no-default-features --features libm --target {target}"
             ),
             "Please fix compiler errors in output above for bevy_mikktspace no_std compatibility.",
-        )
-        ]
+        ));
+
+        commands
     }
 }