Use -DGZ_SANITIZER to fix deprecation warning

[scpeters]

The Garden asan builds have cmake deprecation warnings due to the use of -DIGN_SANITIZER:

• https://build.osrfoundation.org/job/ignition_cmake-ci_asan-main-focal-amd64/15/

CMake Deprecation Warning at cmake/GzSanitizers.cmake:58 (message):
  Ign prefixed variable [IGN_SANITIZER] is deprecated! Use [GZ_SANITIZER]
  instead!
Call Stack (most recent call first):
  cmake/GzCMake.cmake:33 (include)
  CMakeLists.txt:19 (include)

To avoid having complex logic in the Jenkins script to determine which variable to use, the GZ_SANITIZER variable is backported to ign-cmake2 in gazebosim/gz-cmake#294.. Once that is merged and released, this PR can be marked ready for review. This pull request is ready for review.

[scpeters]

Testing using the following deployment:

• https://build.osrfoundation.org/job/_dsl_ignition/1388/

Test results before this branch

• https://build.osrfoundation.org/job/ignition_math-ci_asan-ign-math6-focal-amd64/14/
• https://build.osrfoundation.org/job/ignition_math-ci_asan-gz-math7-focal-amd64/3/

Test results with this branch

• https://build.osrfoundation.org/job/ignition_math-ci_asan-ign-math6-focal-amd64/16/
• https://build.osrfoundation.org/job/ignition_math-ci_asan-gz-math7-focal-amd64/5/