Pin cargo-xbuild to 0.5.29

The new handling of LTO options in 0.5.30+ breaks building on older
toolchains.

Author: crawfxrd

scripts/deps.sh

@@ -51,7 +51,7 @@ rustup component add --toolchain "$(cat rust-toolchain)" rust-src
 if ! which cargo-xbuild > /dev/null
 then
   msg "Installing cargo-xbuild"
-  cargo +nightly install cargo-xbuild
+  cargo +nightly install cargo-xbuild --version=0.5.29
 fi
 
 msg "\x1B[32mSuccessfully installed dependencies"