mlr: TZ environment variable appears malformed when using filename for TZ env var

[DmitriK]

Glibc specifies that the TZ variable can be in the form of :characters (note the colon, also see end of https://www.gnu.org/software/libc/manual/html_node/TZ-Variable.html about third form) where characters is the name of a timezone file. However, running mlr with such an env variables results in an error message:

$ env TZ=:/etc/localtime mlr --csv tail                                                                                                                                                                
mlr: TZ environment variable appears malformed: ":/etc/localtime"

Note that I set this system-wide due to https://blog.packagecloud.io/set-environment-variable-save-thousands-of-system-calls/, so it affects every use of mlr, not just individual scripts.