azuredatastudio: fix file dialogs
File open dialogs in Azure Data Studio cause the application to crash.
As documented in #225989, this can be worked around by setting
XDG_DATA_DIRS, but the correct fix is to use GApps hooks. Add those to
the program packaging, and move the custom fixup phase to a pre-fixup
hook, so the main hooks also get run.
Fixes #225989