ci: downgrade all workflows to ubuntu to 20.04

Author: notmandatory

.github/workflows/code_coverage.yml

@@ -14,7 +14,7 @@ jobs:
   Codecov:
     name: Code Coverage
     if: false # disabled
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     env:
       RUSTFLAGS: "-Cinstrument-coverage"
       RUSTDOCFLAGS: "-Cinstrument-coverage"

.github/workflows/cont_integration.yml

@@ -14,7 +14,7 @@ jobs:
 
   build-test:
     name: Build and test
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         rust:
@@ -77,7 +77,7 @@ jobs:
 
   test-readme-examples:
     name: Test README.md examples
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: checkout
         uses: actions/checkout@v4
@@ -167,7 +167,7 @@ jobs:
 
   fmt:
     name: Rust fmt
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Checkout
         uses: actions/checkout@v4

.github/workflows/nightly_docs.yml

@@ -13,7 +13,7 @@ on:
 jobs:
   build_docs:
     name: Build docs
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Checkout sources
         uses: actions/checkout@v4
@@ -43,7 +43,7 @@ jobs:
     name: 'Publish docs'
     if: github.ref == 'refs/heads/master'
     needs: [build_docs]
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Checkout `bitcoindevkit.org`
         uses: actions/checkout@v4