Add suport for GHC nightlies via GHCup (#279)

This is supported since GHCup 0.1.19.3. See:
https://www.haskell.org/ghcup/guide/#nightlies

.github/workflows/workflow.yml

@@ -101,6 +101,13 @@ jobs:
               ghc:   "9.6.0.20230111"
               cabal: "3.8"
 
+          # Test ghc nightly
+          - os: ubuntu-latest
+            ghcup_release_channel: "https://ghc.gitlab.haskell.org/ghcup-metadata/ghcup-nightlies-0.0.7.yaml"
+            plan:
+              ghc:   "latest-nightly"
+              cabal: "latest"
+
           # setup does something special for 7.10.3 (issue #79)
           - os: ubuntu-20.04
             plan:
@@ -159,8 +166,18 @@ jobs:
           GHCVER="$(ghc --numeric-version)"
           echo "CABALVER=${CABALVER}" >> "${GITHUB_ENV}"
           echo "GHCVER=${GHCVER}"     >> "${GITHUB_ENV}"
+          if [[ "${{ steps.setup.outputs.ghc-version }}" == "latest-nightly" ]]
+          then
+            GHCVER_EXPECTED=$( \
+              curl "${{ matrix.ghcup_release_channel }}" | \
+              yq '.ghcupDownloads.GHC[] | select(.viTags[] | contains("LatestNightly")) | key' \
+            )
+            echo "Latest nightly: ${GHCVER_EXPECTED}"
+          else
+            GHCVER_EXPECTED="${{ steps.setup.outputs.ghc-version }}"
+          fi
           [[ "${CABALVER}" == "${{ steps.setup.outputs.cabal-version }}" ]] && \
-            [[ "${GHCVER}" == "${{ steps.setup.outputs.ghc-version }}" ]]
+            [[ "${GHCVER}" == "${GHCVER_EXPECTED}" ]]
 
       - name: Test runghc
         run: |

setup/src/installer.ts

@@ -8,7 +8,7 @@ import {ghcup_version, OS, Tool, releaseRevision} from './opts';
 import process from 'process';
 import * as glob from '@actions/glob';
 import * as fs from 'fs';
-import {compareVersions} from 'compare-versions'; // compareVersions can be used in the sense of >
+import {compareVersions, validate} from 'compare-versions'; // compareVersions can be used in the sense of >
 
 // Don't throw on non-zero.
 const exec = async (cmd: string, args?: string[]): Promise<number> =>
@@ -171,7 +171,12 @@ export async function installTool(
         await ghcupGHCHead();
         break;
       }
-      if (tool === 'ghc' && compareVersions('8.3', version)) {
+      // “version” may not be a semantic version (e.g. “latest-nightly”),
+      // so guard “compareVersions” with “validate”.
+      if (
+        tool === 'ghc' &&
+        (!validate(version) || compareVersions('8.3', version))
+      ) {
         // Andreas, 2022-12-09: The following errors out if we are not ubuntu-20.04.
         // Atm, I do not know how to check whether we are on ubuntu-20.04.
         // So, ignore the error.