Align seqcli's environment override handling with Seq's

[nblumhardt]

Fixes #365

The README shows how environment variable setting overrides should work, but this was committed in error, the README updates should have gone to Seq Forwarder, which was the project that gained environment override support at that time.

Instead of fixing the README, I've lined up seqcli's configuration handling with Seq's - this should help us along towards a fully-consistent experience across client and server in the future.

SEQCLI_CONNECTION_SERVERURL now works:

seqcli> SEQCLI_CONNECTION_SERVERURL=https://test.example.com seqcli log -m test 
The command failed: nodename nor servname provided, or not known (test.example.com:443) nodename nor servname provided, or not known
seqcli> seqcli log -m test
The command failed: Connection refused (localhost:5341) Connection refused

Sorry about the confusing experience, @richard-stafflink - my mistake, here :-)

[rj-au]

Nice one! thank you for the quick turn-around 🚀