prepare_log_file: update log file creation to use umask for permissions

[Martin-Zeithaml]

1. User can read and write
2. Group can read - do we want to allow write?
3. Others no read | write

chmod removed as the file is created with umask & touch.

Used for zwe command, when --log-dir is specified.

./zwe some_command --config zowe.yaml --log-dir /myLogs

ls -al /myLogs

-rw-rw-rw-   1 MARTIN   ZOWEGRP      157 Jun 12 11:05 zwe-some-command-20260612T110543.log  <- old
-rw-r-----   1 MARTIN   ZOWEGRP      157 Jun 12 11:24 zwe-some-command-20260612T112442.log  <- new

Note: prepareLogFile (typescript version) seems to be not used and there is a bug.

[github-actions]

build 10529 SUCCEEDED.
Link to workflow run: https://github.com/zowe/zowe-install-packaging/actions/runs/27425788964

[github-actions]

Test workflow 8731 is started.
Running install test: Convenience Pax
The zowe artifact being used by this test workflow: libs-snapshot-local/org/zowe/3.5.0-PR-4807/zowe-3.5.0-pr-4807-10529-20260612153744.pax
Running on machine: zzow10
Result: FAILURE
Link to workflow run: https://github.com/zowe/zowe-install-packaging/actions/runs/27426100970

[github-actions]

build 10573 FAILED.
Link to workflow run: https://github.com/zowe/zowe-install-packaging/actions/runs/27742105708

[github-actions]

build 10574 FAILED.
Link to workflow run: https://github.com/zowe/zowe-install-packaging/actions/runs/27742557113