Use Ubuntu 20.04 for builds

ubuntu-latest now points to 22.04:
https://github.blog/changelog/2022-11-09-github-actions-ubuntu-latest-workflows-will-use-ubuntu-22-04/

This means that distros witih older glibc versions can't use the 0.39
binaries attached to the release. The next release should resolve
this (see #446).

.github/workflows/coverage.yml

@@ -4,7 +4,7 @@ on: [pull_request, push]
 
 jobs:
   coverage:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
 

.github/workflows/release.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   push_crates_io:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
       - uses: katyo/publish-crates@v1
@@ -15,7 +15,7 @@ jobs:
           registry-token: ${{ secrets.CARGO_REGISTRY_TOKEN }}
 
   create-release:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
       - uses: taiki-e/create-gh-release-action@v1
@@ -28,11 +28,11 @@ jobs:
       matrix:
         include:
           # - target: aarch64-unknown-linux-gnu
-          #   os: ubuntu-latest
+          #   os: ubuntu-20.04
           # - target: aarch64-apple-darwin
           #   os: macos-latest
           - target: x86_64-unknown-linux-gnu
-            os: ubuntu-latest
+            os: ubuntu-20.04
           - target: x86_64-apple-darwin
             os: macos-latest
           - target: x86_64-pc-windows-msvc

.github/workflows/test.yml

@@ -5,7 +5,7 @@ name: Continuous integration
 jobs:
   test_linux:
     name: Test Linux
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@1.57.0
@@ -13,7 +13,7 @@ jobs:
 
   test_linux_latest_stable:
     name: Test Linux (latest stable Rust)
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@stable
@@ -21,7 +21,7 @@ jobs:
 
   # test_linux_arm64:
   #   name: Test Linux arm64 (build only)
-  #   runs-on: ubuntu-latest
+  #   runs-on: ubuntu-20.04
   #   steps:
   #     - uses: actions/checkout@v3
   #     - uses: dtolnay/rust-toolchain@1.57.0
@@ -47,7 +47,7 @@ jobs:
 
   regression_test:
     name: Output Regression Test
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@1.57.0
@@ -58,7 +58,7 @@ jobs:
 
   package:
     name: Check Linux Packaging
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@1.57.0
@@ -66,7 +66,7 @@ jobs:
 
   fmt:
     name: Rustfmt
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@1.57.0