build: do not reinstall master toolchain if it is up-to-date

Author: tesuji

setup-toolchain.sh

@@ -9,5 +9,11 @@ if [[ "$CI" == true ]] || ! command -v rustup-toolchain-install-master > /dev/nu
   cargo install -Z install-upgrade rustup-toolchain-install-master --bin rustup-toolchain-install-master
 fi
 
-rustup-toolchain-install-master -f -n master
+RUST_COMMIT=$(git ls-remote https://github.com/rust-lang/rust master | awk '{print $1}')
+
+if rustc +master -Vv 2>/dev/null | grep -q "$RUST_COMMIT"; then
+    exit 0
+fi
+
+rustup-toolchain-install-master -f -n master "$RUST_COMMIT"
 rustup override set master