Update mirror.sh to fix DELAY Override

[aristo9999999999]

This primarily fix situation where DELAY override is incorrect.

Previous behaviors:

• If DELAY is unset, will set DELAY_ARG to "--delay=" (empty string after equal sign)
• If DELAY is set, DELAY_ARG will be set to "--delay=10000" if NVD_API_KEY unspecified. DELAY value will never be used.

Changed Behavior

• If DELAY is unset,
  • if NVD_API_KEY unspecified, DELAY_ARG will be set to "--delay=10000"
  • if NVD_API_KEY specified, DELAY_ARG will be empty string (I assume will default to value in CLI)
• If DELAY is set , for example to "1234", DELAY_ARG will be set to "--delay=1234"

[jeremylong]

thanks!