don't ever change the call_button text silly

[tlambert03]

maybe addresses #179 ? (will close separately once confirmed)

[codecov]

Codecov Report

Merging #180 (b0b0c63) into master (c4294a3) will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #180      +/-   ##
==========================================
+ Coverage   90.05%   90.12%   +0.07%     
==========================================
  Files          29       29              
  Lines        2855     2855              
==========================================
+ Hits         2571     2573       +2     
+ Misses        284      282       -2     

Impacted Files	Coverage Δ
magicgui/widgets/_function_gui.py	96.87% <ø> (+0.62%)	⬆️
magicgui/type_map.py	91.13% <0.00%> (+0.63%)	⬆️

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