CMake plugin commands do not seem to pass environment to cmake command.

withEnv(['CC=clang', 'CXX=clang++' {
  cmakeBuild(
    buildDir: 'build/debug',
    buildType: 'Debug',
    generator: 'Ninja',
    installation: 'InSearchPath',
  )
}

[debug] $ docker exec --workdir /var/lib/jenkins/workspace/job/build/debug 67363250bd775405c7cdf92b70a6d7a70dc136a48c93e350fea9cd12ba50d432 cmake -G Ninja -D CMAKE_BUILD_TYPE=Debug /var/lib/jenkins/workspace/job
-- The C compiler identification is GNU 7.2.0
-- The CXX compiler identification is GNU 7.2.0
-- Check for working C compiler: /usr/sbin/cc
-- Check for working C compiler: /usr/sbin/cc – works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/sbin/c++
-- Check for working CXX compiler: /usr/sbin/c++ – works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done

Also, if CMake is installed in /usr/local/bin (and it works in sh step) the CMake builder plugin apparently does not find cmake binary:

[debug] $ cmake -G Ninja -D CMAKE_BUILD_TYPE=Debug /Users/jenkins/workspace/job
Cannot run program "cmake" (in directory "/Users/jenkins/workspace/job/build/debug"): error=2, No such file or directory

In sh step which cmake prints /usr/local/bin/cmake and you can run cmake succesfully