Move rustup component installation to rust-toolchain

This allows cargo check to function correctly without having to first
run prepare_build.sh. cg_clif has been using rust-toolchain too for a
while now.

prepare_build.sh

@@ -1,5 +1,4 @@
 #!/bin/bash --verbose
 set -e
 
-rustup component add rust-src rustc-dev llvm-tools-preview
 ./build_sysroot/prepare_sysroot_src.sh

rust-toolchain

@@ -1 +1,3 @@
-nightly-2021-12-30
+[toolchain]
+channel = "nightly-2021-12-30"
+components = ["rust-src", "rustc-dev", "llvm-tools-preview"]

test.sh

@@ -145,7 +145,7 @@ function test_rustc() {
     echo
     echo "[TEST] rust-lang/rust"
 
-    rust_toolchain=$(cat rust-toolchain)
+    rust_toolchain=$(cat rust-toolchain | grep channel | sed 's/channel = "\(.*\)"/\1/')
 
     git clone https://github.com/rust-lang/rust.git || true
     cd rust