Fix Corso panics in Docker images (#2543)

## Description

The `KOPIA_CACHE_DIRECTORY` env variable causes Corso to panic even if `$CORSO_HOME/kopia/cache` exists.
Removing it fixes the builds but I am unsure of any side effects.

## Does this PR need a docs update or release note?

- [x] ✅ Yes, it's included

## Type of change

- [x] 🐛 Bugfix

## Issue(s)

* #2529 

## Test Plan

- [x] 💪 Manual

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Support for item.Attachment:Mail restore
 - Errors from duplicate names in Exchange Calendars
 - Resolved an issue where progress bar displays could fail to exit, causing unbounded CPU consumption.
+- Fix Corso panic within Docker images
 
 ### Changed
 - When using Restore and Details on Exchange Calendars, the `--event-calendar` flag can now identify calendars by either a Display Name or a Microsoft 365 ID.

build/Dockerfile

@@ -22,7 +22,6 @@ ENV CORSO_HOME=/app/corso
 ENV CORSO_CONFIG_DIR=$CORSO_HOME \
 	KOPIA_CONFIG_PATH=$CORSO_HOME/kopia/config/repository.config \
 	KOPIA_LOG_DIR=$CORSO_HOME/kopia/logs \
-	KOPIA_CACHE_DIRECTORY=$CORSO_HOME/kopia/cache \
 	RCLONE_CONFIG=$CORSO_HOME/kopia/rclone/rclone.conf \
 	KOPIA_PERSIST_CREDENTIALS_ON_CONNECT=false \
 	KOPIA_CHECK_FOR_UPDATES=false