Build particular Rust version

[Matthew James Kraai]

Hi,

Is it possible to build Rust 1.58.1 without updating the `rust`
definition?  If so, how do I do so?

If I apply this patch:

```
diff --git a/gnu/packages/rust.scm b/gnu/packages/rust.scm
index 67dc5cdaf3..61d6686b64 100644
--- a/gnu/packages/rust.scm
+++ b/gnu/packages/rust.scm
@@ -644,10 +644,14 @@ (define rust-1.56
    rust-1.55 "1.56.1" "04cmqx7nn63hzz7z27b2b0dj2qx18rck9ifvip43s6dampx8v2f3"))
 
 (define rust-1.57
+  (rust-bootstrapped-package
+   rust-1.56 "1.57.0" "06jw8ka2p3kls8p0gd4p0chhhb1ia1mlvj96zn78n7qvp71zjiim"))
+
+(define rust-1.58
   (let ((base-rust
          (rust-bootstrapped-package
-          rust-1.56 "1.57.0"
-          "06jw8ka2p3kls8p0gd4p0chhhb1ia1mlvj96zn78n7qvp71zjiim")))
+          rust-1.57 "1.58.1"
+          "1iq7kj16qfpkx8gvw50d8rf7glbm6s0pj2y1qkrz7mi56vfsyfd8")))
     (package
       (inherit base-rust)
       (outputs (cons "rustfmt" (package-outputs base-rust)))
```

and run `./pre-inst-env guix build rust@1.58.1`, it outputs

```
guix build: error: rust: package not found for version 1.58.1
```

-- 
Matt