It was introduced in 4b51f340c8
to prevent accessing user home during tests:
The only place where this the home is currently accessed is when loading
SIP accounts and these can already be overriden with
CALLS_SIP_ACCOUNT_FILE.
It seems that XDG_CONFIG_HOME is propagated to the flatpak runner,
even though it was only overriden for the test suite.
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Building module calls in /builds/devrtz/calls/.flatpak-builder/build/calls-1
========================================================================
Error: module calls: Error opening directory '/builds/devrtz/calls/_build/calls': Permission denied
Reported at https://gitlab.gnome.org/Infrastructure/GitLab/-/issues/596#note_1654082
d6619d68c9