fix: change file permissions from 0600 to 0644 in InstallModel (#8657)

Closes #8119

When installing models from the gallery, files are created with 0600
permissions (owner read/write only), making them unreadable by the
LocalAI server when running as a different user.

This fix changes the permissions to 0644 (owner read/write, group/others
read), allowing the server to read model files regardless of the user
it runs as.

Co-authored-by: localai-bot <localai-bot@users.noreply.github.com>