subscriber: mark builders as must_use (#2239)

## Motivation

Builders not marked `#[must_use]` can not be initialized sometimes, causing silent failures.
Eg.
```rust
fn main() {
    tracing_subscriber::fmt();
    tracing::info!("hello");
}
```
won't print anything.

## Solution

Added `#[must_use]` to builder types in the tracing-subscriber crate.

tracing-subscriber/src/filter/env/builder.rs

@@ -11,6 +11,7 @@ use tracing::level_filters::STATIC_MAX_LEVEL;
 ///
 /// [builder]: https://rust-unofficial.github.io/patterns/patterns/creational/builder.html
 #[derive(Debug, Clone)]
+#[must_use]
 pub struct Builder {
     regex: bool,
     env: Option<String>,

tracing-subscriber/src/fmt/mod.rs

@@ -240,6 +240,7 @@ pub type Formatter<N = format::DefaultFields, E = format::Format, W = fn() -> io
 /// Configures and constructs `Collector`s.
 #[derive(Debug)]
 #[cfg_attr(docsrs, doc(cfg(all(feature = "fmt", feature = "std"))))]
+#[must_use]
 pub struct CollectorBuilder<
     N = format::DefaultFields,
     E = format::Format,