Fix cross-compilation workflow

[traversaro]

🦟 Bug fix

Fixes the failure we experienced in conda-forge/gz-msgs-feedstock#3 .

Summary

The cross-compilation workflow originally added in #60 and documented in

gz-msgs/CMakeLists.txt

Line 32 in 2356549

# Cross-compilation related options

did not work after in #256 the use of ${GZ_MSGS_GEN_EXECUTABLE} was changed back to directly use $<TARGET_FILE:gz_msgs_gen>.

This PR restores the cross-compilation workflow, and it has been tested successfully in conda-forge/gz-msgs-feedstock#3 .

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #319 (d3fc146) into gz-msgs9 (2356549) will not change coverage.
The diff coverage is n/a.

❗ Current head d3fc146 differs from pull request most recent head 55c4b13. Consider uploading reports for the commit 55c4b13 to get more accurate results

@@            Coverage Diff            @@
##           gz-msgs9     #319   +/-   ##
=========================================
  Coverage     95.41%   95.41%           
=========================================
  Files            10       10           
  Lines          1026     1026           
=========================================
  Hits            979      979           
  Misses           47       47           

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