fix: give mysqlsh a writable home dir

[Christos Vontas (cv711)]

This pull request introduces improvements to error handling in snapshot operations and enhances the reliability and isolation of the MySQL Shell environment setup. The main changes are the introduction of a utility for joining snapshot errors with special handling for context cancellation, and the creation of a temporary and isolated environment for MySQL Shell executions. Comprehensive tests have been added to ensure the correctness of these changes.

Snapshot error handling improvements:

• Added joinSnapshotErrors in internal/snapshot/errors.go to aggregate errors, prioritizing non-canceled errors and only including context.Canceled if it is the sole error.
• Updated CreateBackup and RestoreBackup in internal/snapshot/s3.go to use joinSnapshotErrors, improving error reporting consistency. [1] [2]
• Added tests in internal/snapshot/errors_test.go to verify the new error joining behavior.

MySQL Shell environment isolation:

• Modified MySQLShell.run in internal/util/mysql_shell.go to create a temporary home directory and .config for each run, ensuring no interference with the user's environment.
• Added buildMySQLShellEnv to override HOME and XDG_CONFIG_HOME in the shell environment, and corresponding tests in internal/util/mysql_shell_env_test.go. [1] [2]
• Defined mysqlShellConfigHomeEnvKey constant for clarity and maintainability.