Fix the release process's latest step (#4055)

* Fix the release process's latest step

The automated release of 0.36.0 was attempted last night but it failed
due to a [failure on CI][bad]. This failure comes about because it was
trying to change the release date of 0.35.0 which ended up not modifying
any fails so `git` failed to commit as no files were changed.

The original bug though was that 0.35.0 was being changed instead of
0.36.0. The reason for this is that the script used
`--sort=-committerdate` to determine the latest branch. I forgot,
though, that with backports it's possible for 0.35.0 to have a more
recent commit date than 0.36.0 (as is currently the case). This commit
updates the script to perform a numerical sort outside of git to get the
latest release branch.

Additionally this adds in some `set -ex` commands for the shell which
should help print out commands as they're run and assist in future
debugging.

[bad]: https://github.com/bytecodealliance/wasmtime/runs/6087188708

* Replace sed with rust

ci/find-latest-release.rs

@@ -0,0 +1,26 @@
+use std::process::Command;
+fn main() {
+    let output = Command::new("git")
+        .arg("for-each-ref")
+        .arg("refs/remotes/origin")
+        .arg("--format")
+        .arg("%(refname)")
+        .output()
+        .unwrap();
+    assert!(output.status.success());
+    let mut releases = std::str::from_utf8(&output.stdout)
+        .unwrap()
+        .lines()
+        .filter_map(|l| l.strip_prefix("refs/remotes/origin/release-"))
+        .filter_map(|l| {
+            let mut parts = l.split('.');
+            let major = parts.next()?.parse::<u32>().ok()?;
+            let minor = parts.next()?.parse::<u32>().ok()?;
+            let patch = parts.next()?.parse::<u32>().ok()?;
+            Some((major, minor, patch))
+        })
+        .collect::<Vec<_>>();
+    releases.sort();
+    let (major, minor, patch) = releases.last().unwrap();
+    println!("{}.{}.{}", major, minor, patch);
+}