gitlab-ci: really build with pipewire master

Apparently git cannot directly clone at an arbitrary commit,
it must be a branch or a tag

.gitlab-ci.yml

@@ -5,8 +5,8 @@ stages:
 
 variables:
   FDO_UPSTREAM_REPO: 'pipewire/wireplumber'
-  # change to build against a different tag/commit/branch of pipewire
-  PIPEWIRE_HEAD: '613824ab0b447ee7307aeef662fccc758b759e0e'
+  # change to build against a different tag or branch of pipewire
+  PIPEWIRE_HEAD: 'master'
 
 # ci-templates as of May 1st 2020
 .templates_sha: &templates_sha 59de540b620c45739871d1a073d76d5521989d11