transpile: update generated nightly to nightly-2023-04-15 (1.70.0) for cargo's sparse registry

The old git-based registry fetching is extremely slow in CI,
and due to stable crates now building on stable but testing their output on nightly,
is very difficult to correctly cache.
Updating the nightly of their output to the earliest to have the sparse registry should fix this.

Author: kkysen

c2rust-transpile/src/build_files/generated-rust-toolchain.toml

@@ -1,3 +1,3 @@
 [toolchain]
-channel = "nightly-2022-08-08"
+channel = "nightly-2023-04-15" # 1.70 for sparse registry
 components = ["rustfmt"]

c2rust-transpile/tests/snapshots.rs

@@ -115,7 +115,7 @@ fn transpile(platform: Option<&str>, c_path: &Path) {
     let rlib_path = format!("lib{crate_name}.rlib");
     let status = Command::new("rustc")
         .args([
-            "+nightly-2022-08-08",
+            "+nightly-2023-04-15",
             "--crate-type",
             "lib",
             "--edition",