update unrolled perf builds detection

the perfbot comment has recently changed

Author: lqd

src/main.rs

@@ -1205,10 +1205,10 @@ impl Config {
             .lines()
             // lines of table with PR builds
             .filter(|l| l.starts_with("|#"))
-            // get the commit link
-            .filter_map(|l| l.split('|').nth(2))
-            // get the commit sha
-            .map(|l| l.split_once('[').unwrap().1.rsplit_once(']').unwrap().0)
+            // get the commit link: it's in the 3rd column, i.e. the 4th pipe-split chunk
+            .filter_map(|l| l.split('|').nth(3))
+            // get the commit sha: the cell has the format "`$sha` ([link]($ci-repo/commit/$sha))"
+            .map(|l| l.split_once('`').unwrap().1.rsplit_once('`').unwrap().0)
             .collect::<Vec<_>>();
         let short_sha = builds
             .iter()