Auto merge of #65202 - pietroalbini:scriptify-ci-config, r=alexcrichton

ci: move most of the prepare config into scripts

This PR moves most of the configuration from the CI yamls into bash scripts, driven by a small Python script (which understands and emulates the two `##vso[` commands we use).

There are two reasons why we'd want to do this:

* Being able to prepare the build environment locally by just running `src/ci/prepare.py` simplifies a lot setting up a local VM similar to CI (software pre-installed in the CI images won't be prepared, but it's a start anyway).
* When we'll switch to GitHub Actions we'll need to either duplicate code in multiple workflows or write a preprocessor. Having all the prepare steps in a single one is going to simplify the implementation of both options.

Along with the move I did a few changes to the actual scripts:

* Mirrored all the remaining external URLs we download (except chocolatey) to the `rust-lang-ci-mirrors` bucket, to increase reliability and reduce the chance of supply chain attacks. I didn't audit and mirror the CI scripts outside this PR though.
* Extracted CI-specific behavior (like issuing `##vso[` commands and detecting the host platform) into `shared.sh` and included it in most of the scripts. This way a switch to another CI provider will be less painful.

It's possible (and easier) to review this commit-by-commit.
r? @alexcrichton
cc @rust-lang/infra

1 files changed, 13 insertions, 0 deletions

diff --git a/src/ci/scripts/switch-xcode.sh b/src/ci/scripts/switch-xcode.sh
new file mode 100755
index 00000000000..2cbb2ddbc70
--- /dev/null
+++ b/src/ci/scripts/switch-xcode.sh
@@ -0,0 +1,13 @@
+#!/bin/bash
+# Switch to XCode 9.3 on OSX since it seems to be the last version that supports
+# i686-apple-darwin. We'll eventually want to upgrade this and it will probably
+# force us to drop i686-apple-darwin, but let's keep the wheels turning for now.
+
+set -euo pipefail
+IFS=$'\n\t'
+
+source "$(cd "$(dirname "$0")" && pwd)/../shared.sh"
+
+if isMacOS; then
+    sudo xcode-select --switch /Applications/Xcode_9.3.app
+fi