move CONFIG_SERVER_URL environment variable name to a constant

[rtuck99]

moves CONFIG_SERVER_URL into a constant as suggested in

• Support configuring the config server URL mx-bluesky#1698

Instructions to reviewer on how to test:

1. Tests pass

Checks for reviewer

• Would the PR title make sense to a scientist on a set of release notes
• If a new device has been added does it follow the standards
• If changing the API for a pre-existing device, ensure that any beamlines using this device have updated their Bluesky plans accordingly
• Have the connection tests for the relevant beamline(s) been run via dodal connect ${BEAMLINE}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.11%. Comparing base (4ba9de1) to head (6f756e4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2051   +/-   ##
=======================================
  Coverage   99.11%   99.11%           
=======================================
  Files         327      327           
  Lines       12810    12812    +2     
=======================================
+ Hits        12697    12699    +2     
  Misses        113      113           

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