update xapian-apt-index only in case of stable release

[alexl83]

Description

update-apt-xapian-index takes an insane amout of time during buid - compounding over dozens of builds you get a nice overhead in compute-time in the hours range

This PR disables update-apt-xapian-index for unstable releases ( $BETA = yes )

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also note any relevant details for your test configuration.

• Built orangepi5-plus desktop with and without BETA=yes, results below

Baseline 1st run ( CLEAN_LEVEL="debs,cache" )
Repeat Build Options [ ./compile.sh opi5plus build BRANCH=edge BUILD_DESKTOP=yes CLEAN_LEVEL=debs,cache DESKTOP_APPGROUPS_SELECTED='browsers chat desktop_tools office remote_desktop' DESKTOP_ENVIRONMENT=gnome DESKTOP_ENVIRONMENT_CONFIG_NAME=config_base RELEASE=bookworm ]
Runtime [ 14:54 min ]

Baseline 2nd run ( CLEAN_LEVEL="debs" )
Repeat Build Options [ ./compile.sh opi5plus build BRANCH=edge BUILD_DESKTOP=yes CLEAN_LEVEL=debs DESKTOP_APPGROUPS_SELECTED='browsers chat desktop_tools office remote_desktop' DESKTOP_ENVIRONMENT=gnome DESKTOP_ENVIRONMENT_CONFIG_NAME=config_base RELEASE=bookworm ]
Runtime [ 5:45 min ]

Baseline 3rd run ( CLEAN_LEVEL="debs" BETA="yes")
Repeat Build Options [ ./compile.sh opi5plus build BETA=yes BRANCH=edge BUILD_DESKTOP=yes CLEAN_LEVEL=debs DESKTOP_APPGROUPS_SELECTED='browsers chat desktop_tools office remote_desktop' DESKTOP_ENVIRONMENT=gnome DESKTOP_ENVIRONMENT_CONFIG_NAME=config_base RELEASE=bookworm ]
Runtime [ 5:04 min ]

Baseline 4th run ( CLEAN_LEVEL="debs,cache" BETA="yes")
Repeat Build Options [ ./compile.sh opi5plus build BETA=yes BRANCH=edge BUILD_DESKTOP=yes CLEAN_LEVEL=debs,cache DESKTOP_APPGROUPS_SELECTED='browsers chat desktop_tools office remote_desktop' DESKTOP_ENVIRONMENT=gnome DESKTOP_ENVIRONMENT_CONFIG_NAME=config_base RELEASE=bookworm ]
Runtime [ 14:14 min ]

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings