Open GApps silently fails to install via Lineage recovery

Expected Behavior

Sideloading the latest Open GApps from Lineage recovery should add Google apps to the active slot.

Current Behavior

The sideload operation completes successfully, but no Google apps are installed.

Here's what happens:

1. The Open GApps installer script attempts but fails to mount the system partition to /system.
2. The failure to mount /system is ignored by the installer script.
3. The installer script attempts but fails to install each Google app to /system (because /system is on a read-only mount).
4. Each failure is ignored by the installer script.
5. The script exits with a 0 exit status so Lineage recovery believes that it completed sucessfully.

Open GApps could fix the installer script, but it currently works in TWRP so users reasonably expect it to also work with Lineage recovery.

Possible Solution

Steps to Reproduce

1. Flash Lineage recovery to slot b:
   1. Reboot into the bootloader
   2. Run fastboot flash boot_b ./lineage-17.1-20200501-recovery-mata.img
   3. Run fastboot set_active b
2. Boot into recovery (slot b)
3. Install LineageOS on slot a:
   1. Select "Apply update"
   2. Select "Apply from ADB"
   3. Run adb sideload ./lineage-17.1-20200501-nightly-mata-signed.zip
4. Reboot into recovery (slot a)
5. Install Open GApps:
   1. Select "Apply update"
   2. Select "Apply from ADB"
   3. Run adb sideload ./open_gapps-arm64-10.0-pico-20200501.zip
6. Select "Factory reset"

/device mata /version lineage-17.1 /date 2020-05-01 /kernel 4.4.220-lineage+ /baseband 2.0.c4-M2.0.10 /mods OpenGApps

I have read the directions.