Fix linkcheck script from getting out of sync.

Author: ehuss

src/tools/linkchecker/linkcheck.sh

@@ -85,11 +85,11 @@ fi
 if [ ! -e "linkchecker/main.rs" ] || [ "$iterative" = "0" ]
 then
     echo "Downloading linkchecker source..."
+    nightly_hash=$(rustc +nightly -Vv | grep commit-hash | cut -f2 -d" ")
+    url="https://raw.githubusercontent.com/rust-lang/rust"
     mkdir linkchecker
-    curl -o linkchecker/Cargo.toml \
-        https://raw.githubusercontent.com/rust-lang/rust/master/src/tools/linkchecker/Cargo.toml
-    curl -o linkchecker/main.rs \
-        https://raw.githubusercontent.com/rust-lang/rust/master/src/tools/linkchecker/main.rs
+    curl -o linkchecker/Cargo.toml ${url}/${nightly_hash}/src/tools/linkchecker/Cargo.toml
+    curl -o linkchecker/main.rs ${url}/${nightly_hash}/src/tools/linkchecker/main.rs
 fi
 
 echo "Building book \"$book_name\"..."