Looks like Azure has updated images recently to install Rust by default,
but that can interfere with our own compiler (for example Cargo's test
suite we think) so be sure to uninstall it before proceeding.
Since #61212 we've been timing out on OSX, and this looks to be because
we're building tools like Cargo and the RLS twice instead of once. This
turns out to be a slight bug in our configuration. CI builders using the
`RUST_CHECK_TARGET` directive actually execute `make all` just before
their acual target. In `make all` we're building a stage2 cargo, and
then in `make dist` we're building a stage1 cargo.
Other builders use `SCRIPT` which provides explicit control over what
`x.py` script, for example, is used to execute the build. This moves
almost all targets to using `SCRIPT` to ensure that we're explicitly
specifying what's being built where. Additionally this updates the logic
of `RUST_CHECK_TARGET` to remove the pre-flight tidy as well as the
pre-flight `make all`. The system LLVM builder (run on PRs) now
explicitly runs tidy first and then runs the rest of the test suite.
* Clean up the `install-windows-build-deps.yml` file and add some more
comments where appropriate.
* Add some comments to `run.yml`
* Don't fast path the `rustfmt` submodule, but we'll take care of that
later if necessary.
This commit is intended to go through and review/refactor the azure
pipelines configuration we have. The major changes are:
* The separate `{windows,macos,linux}.yml` files are now all merged into
one `run.yml`. This allows a shared "master flow" for all platforms
with divergence only where necessary.
* Some install steps have been separated as `install-*.yml` scripts,
where each script internally matches on the appropriate OS and then
delegates accordingly.
* Some various bits and pieces of cruft have been removed which were
artifacts of Travis's setup or similar.
