Add run_script to build options

[slan]

When iterating it's convenient to have the project files updated without running the full synth/impl loop. This PR propagates the top-level run_script argument to the execution phase.

[codecov]

Codecov Report

Merging #598 (3db36a6) into master (f7c2b94) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #598   +/-   ##
=======================================
  Coverage   81.50%   81.50%           
=======================================
  Files          49       49           
  Lines        6461     6461           
  Branches     1287     1287           
=======================================
  Hits         5266     5266           
  Misses       1007     1007           
  Partials      188      188           

Impacted Files	Coverage Δ
nmigen/build/plat.py	27.51% <0.00%> (ø)

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[whitequark]

If you want to customize the build process in this way I suggest passing do_build=False and then using the build plan yourself.

[slan]

Makes a lot more sense indeed, thanks.