gitlab-ci: do not rely on static paths for the gitlab runner

Before, this assumed that the gitlab-runner was running as the 'fdroid'
user.  Now, it shouldn't matter which user it is running as.

.gitlab-ci.yml

@@ -87,8 +87,8 @@ fdroid-buildserver:
     - virsh -c qemu:///system destroy builder_default || true
     - virsh -c qemu:///system undefine builder_default
           --nvram --managed-save --remove-all-storage --snapshots-metadata || true
-    - git -C /home/fdroid/fdroidserver reset --hard
-    - git -C /home/fdroid/fdroidserver checkout -B master origin/master
+    - git -C $HOME/fdroidserver reset --hard
+    - git -C $HOME/fdroidserver checkout -B master origin/master
     - export PATH="$HOME/fdroidserver:$PATH"
     - if [ "$CI_PROJECT_NAMESPACE" = "fdroid" ]; then
           export head=HEAD^^^;