Use dedicated fixed position admin message for --configure

[ianmcorvidae]

This code wasn't updated when --setlat --setlon and --setalt were, but should use the same method.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 60.64%. Comparing base (3afb294) to head (1013696).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
meshtastic/__main__.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #694      +/-   ##
==========================================
+ Coverage   60.57%   60.64%   +0.06%     
==========================================
  Files          24       24              
  Lines        3777     3773       -4     
==========================================
  Hits         2288     2288              
+ Misses       1489     1485       -4     

Flag	Coverage Δ
unittests	60.64% <0.00%> (+0.06%)	⬆️

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