Cache jom in CI and harden install script

Add jom archive caching to GitHub Actions for both Qt6 x64 and ARM64 build jobs, so install_jom.cmd can reuse jom_1_1_4.zip across runs. Update install_jom.cmd to use explicit repo/Qt paths, only download when the zip is missing, create the target directory when needed, run under cmd, and fail fast on download/extract/install errors.