build: pin ubuntu 22.04 for coverage workflow

This ought to fix the "Unable to locate package kcov" problem

Author: zenith391

.github/workflows/ci.yml

@@ -4,7 +4,7 @@ jobs:
   ci:
     strategy:
       matrix:
-        platform: [ubuntu-latest, windows-latest]
+        platform: [ubuntu-22.04, windows-latest]
     runs-on: ${{ matrix.platform }}
     steps:
       - uses: actions/checkout@v3