arglib is the simple idea of passing structured data via a
conventional environment variable instead of implementing an optparser
for every little tool.
Pop the envvar, decode the contents, return the contents.
Change-Id: Ie44148293a58aae9a0a613895176227d43b491bb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2449
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>