Tweaks base on the old Krill coverage workflow.

.github/workflows/main.yml

@@ -1,36 +1,36 @@
 name: coverage
 
 on:
-  push:
-    branches:
-      main
-
-  pull_request:
+  workflow_run:
+    workflows: ["ci"]
+    types:
+      - completed
 
   workflow_dispatch:
 
-env:
-  CARGO_TERM_COLOR: always
-
 jobs:
-  build:
-    runs-on: self-hosted
+  coverage:
+    name: coverage
+    if: ${{ github.event.workflow_run.conclusion == 'success' }}
     steps:
-      - uses: actions/checkout@v3
+      - name: Checkout repository
+        uses: actions/checkout@v3
+        with:
+          # at the time of writing workflow_run.head_commit.id appears to be
+          # the commit that triggered the CI workflow which then triggered us.
+          ref: ${{ github.event.workflow_run.head_commit.id }}
 
       - name: Ensure correct Rust toolchain version
         uses: actions-rs/toolchain@v1
         with:
           toolchain: stable
+
+      - run: cargo install cargo-tarpaulin
 
-      - name: Run cargo-tarpaulin
-        uses: actions-rs/[email protected]
-        with:
-          version: '0.27.1'
-          out-type: Html
+      - run: cargo tarpaulin --verbose --out Html
 
       - name: Archive code coverage results
-        uses: actions/upload-artifact@v1
+        uses: actions/upload-artifact@v2
         with:
           name: code-coverage-report
           path: tarpaulin-report.html