groot: fix go:generate portability for root-gen-type

[dolmen]

Use go run instead of requiring the binary to be already built.

[codecov-commenter]

Codecov Report

Merging #952 (01ac338) into main (8c3e3fa) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #952   +/-   ##
=======================================
  Coverage   72.57%   72.57%           
=======================================
  Files         410      410           
  Lines       62067    62067           
=======================================
  Hits        45044    45044           
  Misses      14692    14692           
  Partials     2331     2331           

Impacted Files	Coverage Δ
groot/cmd/root-gen-type/main.go	20.65% <ø> (ø)

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[sbinet]

LGTM