bump-stage0: pick highest common toml version, add a workaround

- We pick the higest common `toml` version used in the r-l/r workspace
  to avoid introducing Yet Another `toml` `0.x` version, which happens
  to be `0.8.23` as of the time of writing.
- We introduce a byte-buffer-to-string workaround for the `toml 0.8.*`
  series that do not have the `toml 0.9.*` series's `toml::from_slice`
  API yet. Not efficient, but this is not perf-critical so it's fine.

Cargo.lock

@@ -336,7 +336,7 @@ dependencies = [
  "curl",
  "indexmap",
  "serde",
- "toml 0.7.8",
+ "toml 0.8.23",
 ]
 
 [[package]]

src/tools/bump-stage0/Cargo.toml

@@ -11,4 +11,4 @@ build_helper = { path = "../../build_helper" }
 curl = "0.4.38"
 indexmap = { version = "2.0.0", features = ["serde"] }
 serde = { version = "1.0.125", features = ["derive"] }
-toml = "0.7"
+toml = "0.8.23"

src/tools/bump-stage0/src/main.rs

@@ -185,7 +185,11 @@ fn fetch_manifest(
         format!("{}/dist/channel-rust-{}.toml", config.dist_server, channel)
     };
 
-    Ok(toml::from_slice(&http_get(&url)?)?)
+    // FIXME: on newer `toml` (>= `0.9.*`), use `toml::from_slice`. For now, we use the most recent
+    // `toml` available in-tree which is `0.8.*`, so we have to do an additional dance here.
+    let response = http_get(&url)?;
+    let response = String::from_utf8(response)?;
+    Ok(toml::from_str(&response)?)
 }
 
 fn http_get(url: &str) -> Result<Vec<u8>, Error> {