Detect ign instead of using cmake module to check for ignition-tools

[j-rivero]

Part of gazebo-tooling/release-tools#472

I don't detect the conditional installation of the ign submodule, the PR just
changed the way of detecting ign for testing.

[codecov]

Codecov Report

Merging #240 (764250d) into ign-gui3 (3df3ad6) will increase coverage by 51.13%.
The diff coverage is 59.81%.

@@              Coverage Diff              @@
##           ign-gui3     #240       +/-   ##
=============================================
+ Coverage     14.55%   65.68%   +51.13%     
=============================================
  Files            13       23       +10     
  Lines          1333     2827     +1494     
=============================================
+ Hits            194     1857     +1663     
+ Misses         1139      970      -169     

Impacted Files	Coverage Δ
include/ignition/gui/Helpers.hh	100.00% <ø> (+100.00%)	⬆️
include/ignition/gui/Plugin.hh	100.00% <ø> (+100.00%)	⬆️
src/ign.cc	37.83% <ø> (+37.83%)	⬆️
src/plugins/grid_3d/Grid3D.cc	28.57% <0.00%> (ø)
src/plugins/image_display/ImageDisplay.cc	23.27% <ø> (ø)
src/plugins/publisher/Publisher.cc	90.78% <ø> (ø)
src/Plugin.cc	57.44% <22.22%> (+57.44%)	⬆️
src/plugins/screenshot/Screenshot.cc	33.33% <33.33%> (ø)
src/plugins/scene3d/Scene3D.cc	42.38% <41.86%> (ø)
src/plugins/key_publisher/KeyPublisher.cc	56.52% <56.52%> (ø)
... and 29 more

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[jennuine]

LGTM