add build.rs that dlds the test resources

To be able to run the unit tests seamlessly on any dev machine, we need
to have the test resources downloaded. This can be achieved by calling
the post-checkout script. Since this does not need to be executed when
running with Buildkite, but all the time, the script is now moved to
.buildkite/download_resources.sh and it is no longer a Buildkite hook,
but rather a regular bash script.

The script is called from build.rs so that it is transparent to the user
when the resources are downloaded. The disadvantage with this approach
is that the resources will be downloaded even when calling cargo build,
and not only for cargo test. This can be updated in the future once
cargo adds support for identifying the build profile in build scripts.
Tracking issue: rust-lang/cargo#4001

Signed-off-by: Andreea Florescu <fandree@amazon.com>

build.rs

@@ -0,0 +1,10 @@
+use std::process::Command;
+
+fn main() {
+    let command = "./.buildkite/download_resources.sh";
+    Command::new(command).status().unwrap();
+
+    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rerun-if-changed=.buildkite/download_resources.sh");
+    println!("cargo:rerun-if-changed=src/loader/x86_64/bzimage/bzimage");
+}