back/vendor: s/ilang/rtlil as a result of YosysHQ/yosys#4704

[vk2seb]

#1540 did not catch all uses of this command that was deprecated upstream in YosysHQ/yosys#4704

[codecov]

Codecov Report

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

Project coverage is 91.26%. Comparing base (ab3a355) to head (a9598c0).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
amaranth/back/verilog.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1542      +/-   ##
==========================================
- Coverage   91.34%   91.26%   -0.09%     
==========================================
  Files          44       44              
  Lines       11431    11431              
  Branches     2222     2222              
==========================================
- Hits        10442    10432      -10     
- Misses        828      833       +5     
- Partials      161      166       +5     

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[whitequark]

Thanks!