log: add a GUM_LOG_TIME environment variable equivalent to --time

[Infinidoge]

Is your feature request related to a problem? Please describe.
I want to use gum log with a consistent display format, however not all parts of that actually have environment variables. Namely: You can't set the time format with an environment variable.

Additional note, may make into its own issue: The gum log examples include gum log -tsl ..., which is an invalid command for the output because it will treat sl as the time format.

Describe the solution you'd like
GUM_LOG_TIME environment variable that is equivalent to setting --time.

Describe alternatives you've considered
I don't believe there is an alternative, as environment variables are the dedicated way for setting stuff persistently like this. Time has style variables, so it not having a corresponding format variable is jarring.

Additional context
Needing to do gum log -t timeonly over and over again led to me making a function to wrap gum log in my script.