[tests-only][full-ci] remove c compiler env on client build stage

[PrajwolAmatya]

Description

This PR removes the envs DCMAKE_C_COMPILER and DCMAKE_CXX_COMPILER from the client build step.

This fixes the error:

-- The CXX compiler identification is unknown
-- The C compiler identification is unknown
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - failed
-- Check for working CXX compiler: /ownbuild/master/linux-64-gcc/bin/clang++
-- Check for working CXX compiler: /ownbuild/master/linux-64-gcc/bin/clang++ - broken
CMake Error at /ownbuild/master/linux-64-gcc/dev-utils/cmake-base/share/cmake-3.31/Modules/CMakeTestCXXCompiler.cmake:73 (message):
  The C++ compiler

    "/ownbuild/master/linux-64-gcc/bin/clang++"

  is not able to compile a simple test program.

  It fails with the following output:

    Change Dir: '/__w/client/client/build/CMakeFiles/CMakeScratch/TryCompile-sYxEpS'
    
    Run Build Command(s): /ownbuild/master/linux-64-gcc/dev-utils/bin/ninja -v cmTC_e2922
    [1/2] /ownbuild/master/linux-64-gcc/bin/clang++    -o CMakeFiles/cmTC_e2922.dir/testCXXCompiler.cxx.o -c /__w/client/client/build/CMakeFiles/CMakeScratch/TryCompile-sYxEpS/testCXXCompiler.cxx
    FAILED: [code=127] CMakeFiles/cmTC_e2922.dir/testCXXCompiler.cxx.o 
    /ownbuild/master/linux-64-gcc/bin/clang++    -o CMakeFiles/cmTC_e2922.dir/testCXXCompiler.cxx.o -c /__w/client/client/build/CMakeFiles/CMakeScratch/TryCompile-sYxEpS/testCXXCompiler.cxx
    /ownbuild/master/linux-64-gcc/bin/clang++: error while loading shared libraries: libxml2.so.16: cannot open shared object file: No such file or directory
    ninja: build stopped: subcommand failed.
    
    

  

  CMake will not be able to correctly generate this project.
Call Stack (most recent call first):
  CMakeLists.txt:11 (project)


-- Configuring incomplete, errors occurred!
Error: Process completed with exit code 1.

Failed builds:

• https://github.com/owncloud/client/actions/runs/21191938915/job/60959932601
• https://github.com/owncloud/client/actions/runs/21154836392/job/60848077447