CI: Include a single Ubuntu 18.04 job

This is done purely for the sake of increasing our coverage of Linux when
making releases. See #1140 for the motivation.

We won't run any of the tests et al. for this job.

.github/ci.sh

@@ -117,14 +117,14 @@ sign() {
 
 zip_dist() {
   : "${VERSION?VERSION is required as an environment variable}"
-  name="${name:-"cryptol-$VERSION-$RUNNER_OS-x86_64"}"
+  name="${name:-"cryptol-$VERSION-$OS_TAG-x86_64"}"
   cp -r dist "$name"
   tar -cvzf "$name".tar.gz "$name"
 }
 
 zip_dist_with_solvers() {
   : "${VERSION?VERSION is required as an environment variable}"
-  name="${name:-"cryptol-$VERSION-$RUNNER_OS-x86_64"}"
+  name="${name:-"cryptol-$VERSION-$OS_TAG-x86_64"}"
   sname="${name}-with-solvers"
   cp "$(which abc)"        dist/bin/
   cp "$(which cvc4)"       dist/bin/

.github/workflows/ci.yml

@@ -68,6 +68,11 @@ jobs:
             ghc-version: 9.0.2
           - os: windows-2022
             ghc-version: 9.2.2
+        include:
+          # We include one job from an older Ubuntu LTS release to increase our
+          # coverage of possible Linux configurations.
+          - os: ubuntu-18.04
+            ghc-version: 8.8.4
     outputs:
       test-lib-json: ${{ steps.test-lib.outputs.targets-json }}
     env:
@@ -165,12 +170,16 @@ jobs:
           NAME="${{ needs.config.outputs.name }}-${{ runner.os }}-x86_64"
           echo "NAME=$NAME" >> $GITHUB_ENV
           .github/ci.sh zip_dist $NAME
+        env:
+          OS_TAG: ${{ matrix.os }}
 
       - shell: bash
         run: |
           NAME="${{ needs.config.outputs.name }}-${{ runner.os }}-x86_64"
           echo "NAME=$NAME" >> $GITHUB_ENV
           .github/ci.sh zip_dist_with_solvers $NAME
+        env:
+          OS_TAG: ${{ matrix.os }}
 
       - if: needs.config.outputs.release == 'true'
         shell: bash

README.md

@@ -75,6 +75,7 @@ Cryptol builds and runs on various flavors of Linux, Mac OS X, and
 Windows. We regularly build and test it in the following environments:
 
 - macOS 10.15, 64-bit
+- Ubuntu 18.04, 64-bit
 - Ubuntu 20.04, 64-bit
 - Windows Server 2019, 64-bit