Upgrade to Rust 1.48.0 in rust-toolchain

A severe performance regression with `async fn` was fixed in 1.48.0,
which greatly improves compilation times on many people's machines.

rust-lang/rust#78410

As such, we bump the `rust-toolchain` file to 1.48.0 stable for a
smoother developer experience, even if 1.46.0 is still technically
supported features-wise.

Author: ebkalderon

README.md

@@ -6,8 +6,8 @@ container engine.
 ## Getting started
 
 To build this project on Linux, you will need the latest version of the
-[Rust toolchain](https://www.rust-lang.org/) (at least version 1.46.0, as
-provided with the `rust-toolchain` file).
+[Rust toolchain](https://www.rust-lang.org/) (at least version 1.46.0, but
+1.48.0 compiles async code faster, as provided in the `rust-toolchain` file).
 
 The following binaries will also need to be available in `$PATH`:
 

rust-toolchain

@@ -1 +1 @@
-1.46.0
+1.48.0