ci: fix coverage reporting via Codecov (#11018)

refs: #10930

## Description

#10930 unintentionally removed some codecov file generation logic which resulted in a drop of overall coverage [here](https://app.codecov.io/github/Agoric/agoric-sdk?search=&trend=7%20days). This PR attempts to fix that by reincorporating those changes. 

### Security Considerations

None

### Scaling Considerations

None

### Documentation Considerations

None

### Testing Considerations

Fixes affected Codecov report

### Upgrade Considerations

None

.github/actions/post-test/action.yml

@@ -18,6 +18,12 @@ inputs:
 runs:
   using: composite
   steps:
+    - name: collect coverage
+      shell: bash
+      if: ${{ github.repository_owner == 'agoric' }} && (success() || failure())
+      continue-on-error: true
+      run: |
+        ./scripts/ci/collect-testruns.sh
     - name: upload coverage prepare
       shell: bash
       id: coverage-prep

scripts/ci/collect-testruns.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+set -o xtrace
+SCRIPT_DIR=$(cd ${0%/*} && pwd -P)
+
+for cov in ./packages/**/coverage; do
+  # skip if unable to expand if there are no test files
+  if [[ $cov != "./packages/**/coverage" ]]; then
+    echo "processing $pkg"
+    (cd "$cov/../" && yarn run c8 report --reporter=lcov && find coverage -name "*.json" -delete)
+  fi
+done
+
+# if root coverage exists, ignore it
+if [[ -d "coverage" ]]; then
+  rm -rf coverage
+fi