Unconditionally find the python interpreter

[mjcarroll]

🦟 Bug fix

Fixes #354

Summary

The python interpreter was only being found with BUILD_TESTING enabled. This makes it an unconditional dependency.

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 #356 (97a188e) into main (5935dcb) will increase coverage by 1.46%.
The diff coverage is 97.85%.

❗ Current head 97a188e differs from pull request most recent head 1eb3581. Consider uploading reports for the commit 1eb3581 to get more accurate results

@@            Coverage Diff             @@
##             main     #356      +/-   ##
==========================================
+ Coverage   95.43%   96.89%   +1.46%     
==========================================
  Files          10       28      +18     
  Lines        1030     1064      +34     
==========================================
+ Hits          983     1031      +48     
+ Misses         47       33      -14     

Impacted Files	Coverage Δ
core/generator/Generator.hh	100.00% <ø> (ø)
core/generator/generator_main.cc	100.00% <ø> (ø)
...re/include/gz/msgs/detail/PointCloudPackedUtils.hh	94.80% <ø> (ø)
core/include/gz/msgs/MessageFactory.hh	80.00% <80.00%> (ø)
core/src/DynamicFactory.cc	82.00% <82.00%> (ø)
...re/include/gz/msgs/convert/SphericalCoordinates.hh	86.95% <86.95%> (ø)
core/src/Factory.cc	87.50% <87.50%> (ø)
core/src/MessageFactory.cc	96.87% <96.87%> (ø)
compiled/src/gz.cc	90.90% <100.00%> (ø)
core/generator/Generator.cc	91.93% <100.00%> (ø)
... and 18 more