fix: bugs in entrypoint scripts

[sjinks]

This pull request introduces improvements to the permission handling scripts in the Lando core plugins, focusing on safer operations and enhanced robustness. The main changes involve adding checks before performing ownership and permission modifications to avoid errors when directories are missing or not writable, and improving the handling of user home directories.

Permission handling improvements:

• Added a check to ensure /helpers is writable before attempting to change file permissions in lando-entrypoint.sh, preventing errors when the directory is not writable.
• Modified perm_sweep() in user-perm-helpers.sh to check if /usr/local/bin exists before running chown, reducing unnecessary errors.
• Consolidated and simplified background ownership sweeps for /usr/local directories, checking for existence before running find commands, and removed redundant sweeps for subdirectories.

User home directory handling:

• Enhanced the sweep for the user's home directory by checking if getent is available, and confirming the directory exists before running ownership changes, improving reliability and avoiding potential failures.

Parameter usage correction:

• Updated the invocation of perm_sweep in user-perms.sh to use the group ID variable directly, ensuring correct parameter passing.