Disable building against the Beta toolchain.

This is more fallout from
rust-lang/rust#129031 -- the lang team appears
to have relented, but removing this lint hasn't been backported into
Beta yet. So we're in a situation where traditional matches for
uninhabited types are fine on both stable and nightly, but failing on
beta.

Not sure what to do except disable beta.

.github/workflows/ci.yml

@@ -58,7 +58,8 @@ jobs:
  toolchain:
  - stable
  - nightly
- - beta
+ #- beta TODO: re-enable once beta stops being broken, first release
+ #after 11 September should do it
  steps:
  - uses: actions/checkout@v4
  - run: rm rust-toolchain.toml
@@ -77,7 +78,7 @@ jobs:
  toolchain:
  - stable
  - nightly
- - beta
+ # - beta TODO: re-enable, see above
  needs: build_future
  runs-on: ubuntu-latest
  steps:
@@ -101,7 +102,7 @@ jobs:
  toolchain:
  - stable
  - nightly
- - beta
+ # - beta TODO: re-enable, see above
  steps:
  - uses: actions/checkout@v4
  - run: rm rust-toolchain.toml