GitHub Actions: include list of GHC versions in matrix

Alongside "include" include the list of GHC versions in the matrix.
That is, instead of:

  matrix:
    include:
      - ghc: 8.10.3
        allow-failures: false
      - ghc: 8.8.4
        allow-failures: false

haskell-ci now produces:

  matrix:
    ghc:
      - 8.10.3
      - 8.8.4
    include:
      - ghc: 8.10.3
        allow-failures: false
      - ghc: 8.8.4
        allow-failures: false

This approach reduces the work required to introduce another
"dimension" into the matrix.  As a concrete example, the /notmuch/
library must be tested against various versions of libnotmuch.  With
the "ghc" version list defined, adding the list of notmuch versions
to test yields a matrix that is the cartesian product of the two (or
more) lists.  The "include" dicts correctly propagate the
"allow-failures" fields to corresponding matrix legs (evidence:
https://github.com/purebred-mua/hs-notmuch/actions/runs/466042178).

Author: frasertweedale

src/HaskellCI/GitHub/Yaml.hs

@@ -115,7 +115,8 @@ instance ToYaml GitHubJob where
             item $ "continue-on-error" ~> fromString continueOnError
         item $ "strategy" ~> ykeyValuesFilt []
             [ "matrix" ~> ykeyValuesFilt []
-                [ "include" ~> ylistFilt [] (map toYaml ghjMatrix)
+                [ "ghc" ~> ylistFilt [] (map (fromString . prettyShow . ghmeGhcVersion) ghjMatrix)
+                , "include" ~> ylistFilt [] (map toYaml ghjMatrix)
                 ]
             , "fail-fast" ~> YBool [] False
             ]