ci: expose distro name in dockerized GitHub jobs

Expose a distro name in dockerized jobs. This will be used in a
subsequent commit where we merge the installation scripts for dockerized
and non-dockerized jobs.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.github/workflows/main.yml

@@ -342,12 +342,16 @@ jobs:
         vector:
         - jobname: linux-musl
           image: alpine
+          distro: alpine-latest
         - jobname: linux32
           image: daald/ubuntu32:xenial
+          distro: ubuntu32-16.04
         - jobname: pedantic
           image: fedora
+          distro: fedora-latest
     env:
       jobname: ${{matrix.vector.jobname}}
+      distro: ${{matrix.vector.distro}}
     runs-on: ubuntu-latest
     container: ${{matrix.vector.image}}
     steps: