update the release process with bump-stage0

This PR updates the release process following up on the addition of the
bump-stage0 tool in the rust-lang/rust monorepo, making these changes:

* Downloading artifacts from dev-static.rust-lang.org is not supported
  anymore, so the step to bump stage0 during beta promotion is removed.
  This doesn't have any practical effect, as we're still building the
  new beta with the previous beta.

* As there is no dependency anymore between promoting beta and promoting
  stable both are moved to the same day, bringing the master bootstrap
  update one day earlier.

* The step to update the beta bootstrap compiler after the release was
  updated to use the tool.

Author: pietroalbini

src/release/process.md

@@ -11,7 +11,12 @@ Mark it as `rollup=never`, because if it lands in a rollup as *not* the first
 PR then other pull requests in that rollup will be incorrectly associated with
 the prior release.
 
-## Promote beta to stable (T-3 days, Monday)
+## Promote branches (T-3 days, Monday)
+
+Both promotions should happen on Monday. You can open both PRs at the same
+time, but make sure the stable promotion lands first.
+
+### Beta to stable
 
 Temporarily turn off GitHub branch protection for the `stable` branch in
 rust-lang/rust repo. In your local Rust repo:
@@ -47,7 +52,7 @@ Test rustup with:
 RUSTUP_DIST_SERVER=https://dev-static.rust-lang.org rustup update stable
 ```
 
-## Promote master to beta (T-2 days, Tuesday)
+### Master to beta
 
 Gather the relevant information and push the new Cargo branch:
 
@@ -70,22 +75,14 @@ git push git@github.com:rust-lang/rust $BRANCH_POINT:beta -f
 ```
 
 Re-enable branch protection on GitHub. Send a PR to the freshly created beta
-branch of rust-lang/rust which:
-
-- Update `src/stage0.txt`
-  - Change `date` to "YYYY-MM-DD" where the date is the archive date the stable
-    build was uploaded
-  - Change `rustc` to "X.Y.Z" where that's the version of rustc you just build
-  - Comment `rustfmt: nightly-YYYY-MM-DD`
-  - Uncomment `dev: 1`
-- Update `src/ci/channel` to `beta`
+branch of rust-lang/rust which updates `src/ci/channel` to `beta`.
 
-## Master bootstrap update (T-1 day, Wednesday)
+## Master bootstrap update (T-2 day, Tuesday)
 
 Send a PR to the master branch to:
 
-- Update `src/stage0.txt` to change `date` to "YYYY-MM-DD" where the date is
-  the archive date when the beta build was uploaded.
+- Run `./x.py run src/tools/bump-stage0` to update the bootstrap compiler to
+  the beta you created yesterday.
 
 - Remove references to the `bootstrap` and `not(bootstrap)` conditional
   compilation attributes. You can find all of them by installing [ripgrep] and
@@ -157,8 +154,9 @@ Decide on a time to do the release, T.
   git push git@github.com:rust-lang/cargo.git $CARGO_VERSION
   ```
 
-- **T+1hr** Send a PR to the beta branch to comment out `dev: 1` again and
-  update the date to download from (modifying `src/stage0.txt`).
+- **T+1hr** Send a PR to the beta branch running `./x.py run
+  src/tools/bump-stage0` to bump the boostrap compiler to the stable you
+  just released.
 
 [update-thanks]: https://github.com/rust-lang/thanks/actions/workflows/ci.yml