chore(ci): Do not test nightly rust on every PR (#1940)

## Description

Normal CI jobs for PRs now only run using the stable rust compiler,
this gives fewer surprises to users.

Instead the beta compiler is run in a new workflow which runs once a
week.  When the beta workflow fails we get a post in a discord channel.

## Notes & open questions

The required checks for branch protection have changed again.  We no
longer need the nightly checks in those.

## Change checklist

- [x] Self-review.

.github/workflows/beta.yaml

@@ -0,0 +1,40 @@
+# Run tests using the beta Rust compiler
+
+name: Beta Rust
+
+on:
+  schedule:
+    # 06:50 UTC every Monday
+    - cron: '50 6 * * 1'
+  workflow_dispatch:
+
+concurrency:
+  group: beta-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  tests:
+    uses: './.github/workflows/tests.yaml'
+    with:
+      rust-version: beta
+  notify:
+    needs: tests
+    if: ${{ always() }}
+    runs-on: ubuntu-latest
+    steps:
+      - name: Extract test results
+        run: |
+          printf '${{ toJSON(needs) }}\n'
+          result=$(echo '${{ toJSON(needs) }}' | jq -r .tests.result)
+          echo TESTS_RESULT=$result
+          echo "TESTS_RESULT=$result" >>"$GITHUB_ENV"
+      - name: Notify discord on failure
+        uses: n0-computer/discord-webhook-notify@v1
+        if: ${{ env.TESTS_RESULT == 'failure' }}
+        with:
+          severity: error
+          details: |
+            Rustc beta tests failed
+            See https://github.com/n0-computer/iroh/actions/workflows/beta.yaml
+          webhookUrl: ${{ secrets.DISCORD_N0_GITHUB_CHANNEL_WEBHOOK_URL }}
+

.github/workflows/tests.yaml

@@ -5,6 +5,10 @@ name: Tests
 on:
   workflow_call:
     inputs:
+      rust-version:
+        description: 'The version of the rust compiler to run'
+        type: string
+        default: 'stable'
       flaky:
         description: 'Whether to also run flaky tests'
         type: boolean
@@ -29,7 +33,7 @@ jobs:
       fail-fast: false
       matrix:
         name: [ubuntu-latest, macOS-arm-latest]
-        rust: [nightly, stable]
+        rust: [ '${{ inputs.rust-version }}' ]
         features: [all, none, default]
         include:
           - name: ubuntu-latest
@@ -52,7 +56,7 @@ jobs:
       with:
         ref: ${{ inputs.git-ref }}
 
-    - name: Install ${{ matrix.rust }}
+    - name: Install ${{ matrix.rust }} rust
       uses: dtolnay/rust-toolchain@master
       with:
         toolchain: ${{ matrix.rust }}
@@ -114,7 +118,7 @@ jobs:
       fail-fast: false
       matrix:
         name: [windows-latest]
-        rust: [nightly, stable]
+        rust: [ '${{ inputs.rust-version}}' ]
         features: [all, none, default]
         target:
           - x86_64-pc-windows-msvc