duneci-install-module: HEAD might not be a symbolic ref

This might happen, for example, when a tag is requested via
`DUNECI_BRANCH`.
14 jobs for master in 68 minutes and 57 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Stage0
passed #3337
pull base distributions

00:00:27

 
  Stage1
passed #3341
base:16.04

00:00:27

passed #3342
base:16.10

00:00:18

passed #3339
base:8

00:00:23

passed #3340
base:9

00:08:18

passed #3338
docker-builder

00:00:18

 
  Stage2
passed #3343
base:8-backports

00:01:12

passed #3344
dune:2.3

00:01:07

passed #3345
dune:2.4

00:01:06

passed #3346
dune:2.5

00:00:45

passed #3347
dune:git

00:26:08

 
  Stage3
passed #3348
dune-fufem:2.4

00:07:11

passed #3349
dune-fufem:git

00:06:48

 
  Publish
passed #3350
publish

00:25:59