Auto merge of #65202 - pietroalbini:scriptify-ci-config, r=<try>

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

src/ci/azure-pipelines/auto.yml

@@ -315,6 +315,7 @@ jobs:
 
       # 32/64 bit MSVC and GNU deployment
       dist-x86_64-msvc:
+        MSYS_BITS: 64
         RUST_CONFIGURE_ARGS: >
           --build=x86_64-pc-windows-msvc
           --target=x86_64-pc-windows-msvc,aarch64-pc-windows-msvc
@@ -324,6 +325,7 @@ jobs:
         DIST_REQUIRE_ALL_TOOLS: 1
         DEPLOY: 1
       dist-i686-msvc:
+        MSYS_BITS: 32
         RUST_CONFIGURE_ARGS: >
           --build=i686-pc-windows-msvc
           --target=i586-pc-windows-msvc