ci: fix thread-sanitizer build with lld and PIC googletest

- tsan build failed at link with GNU ld:
  "failed to set dynamic section sizes: bad value" (known binutils +
  ThreadSanitizer incompatibility); install lld and select it via
  -fuse-ld=lld for the tsan job only
- pass -fuse-ld=lld through cxx-flags so it reaches the link line,
  avoiding the semicolon-list pitfall of
  list(APPEND CMAKE_EXE_LINKER_FLAGS ...)
- build the bundled googletest with CMAKE_POSITION_INDEPENDENT_CODE=ON:
  lld rejects R_X86_64_32 relocations from the non-PIC libgtest.a when
  producing the position-independent tsan executable; the bundle is built
  by a separate cmake invocation, so the flag must be set there

cmake/FairMQBundlePackageHelper.cmake

@@ -68,6 +68,7 @@ function(build_bundled package bundle)
 
     exec(${CMAKE_COMMAND} -S ${${package}_SOURCE_DIR} -B ${${package}_BINARY_DIR} -G ${CMAKE_GENERATOR}
       -DCMAKE_INSTALL_PREFIX=${${package}_INSTALL_DIR} -DBUILD_GMOCK=OFF
+      -DCMAKE_POSITION_INDEPENDENT_CODE=ON
     )
     exec(${CMAKE_COMMAND} --build ${${package}_BINARY_DIR})
     exec(${CMAKE_COMMAND} --build ${${package}_BINARY_DIR} --target install)