CI: Give up debug builds and add beta builds in test jobs.

The `coverage` job already tests debug mode on Rust Nightly. We were still
missing `beta` coverage, so add it. Also document this.

Author: elardus-erasmus

.github/workflows/ci.yml

@@ -160,16 +160,18 @@ jobs:
           - x86_64-unknown-linux-gnu
 
         mode:
-          - # debug
+          # `coverage` jobs build in debug mode, so only build in release mode here.
           - --release
 
         rust_channel:
           - stable
           # Keep in sync with Cargo.toml and similar `rust_channel` sections.
           - 1.61.0 # MSRV
-          # TODO: Move these to a daily/pre-release job.
+          # Early warning for changes in the Rust toolchain that are about to
+          # break builds on Rust Stable.
+          - beta
+          # XXX: `coverage` jobs run on Nightly.
           # - nightly
-          # - beta
 
         include:
           - target: aarch64-apple-darwin
@@ -539,10 +541,12 @@ jobs:
           - x86_64-unknown-linux-musl      # Has assembly
 
         mode:
+          # This is the main coverage we have for debug mode builds.
           - # debug
 
         # Coverage collection is Nightly-only
         rust_channel:
+          # This is the main coverage we have for building with Rust Nightly.
           - nightly
 
         # TODO: targets