CI cmake-based builds are not using ccache even if specified in build spec by 'ccache' : true

[janvrany]

While working on RISC-V native builds, I noticed that builds that use CMake do not use ccache even if specified.

For example, RISC-V native build, ccache is true [1], but looking at RISC-V build [2] shows
that initialy cache is empty:

16:18:13  Output CCACHE stats before running and clear them
[Pipeline] sh
16:18:13  + ccache -s -z
16:18:14  Local storage:
16:18:14    Cache size (GiB): 0.0 / 5.0 ( 0.00%)
16:18:14  Statistics zeroed

CMake seems to use /usr/bin/c++ and /usr/bin/cc directly (i.e., not ccache wrappers not using ccache to launch the compiler):

16:18:16  -- Check for working CXX compiler: /usr/bin/c++ - skipped
16:18:16  -- Detecting CXX compile features
16:18:16  -- Detecting CXX compile features - done
16:18:16  -- Detecting C compiler ABI info
16:18:17  -- Detecting C compiler ABI info - done
16:18:17  -- Check for working C compiler: /usr/bin/cc - skipped

And indeed cache is still completely empty after the build:

[Pipeline] echo
17:24:57  Output CCACHE stats after running
[Pipeline] sh
17:24:57  + ccache -s
17:24:57  Local storage:
17:24:57    Cache size (GiB): 0.0 / 5.0 ( 0.00%)
[Pipeline] }

Same happens for other builds too as far as I can see: [3], [4].

Interestingly, for some builds it does work, for example linux_ppc-64_le_gcc [5]:

14:51:48  -- The CXX compiler identification is GNU 9.4.0
14:51:48  -- The C compiler identification is GNU 9.4.0
14:51:48  -- Check for working CXX compiler: /usr/lib/ccache/c++
14:51:48  -- Check for working CXX compiler: /usr/lib/ccache/c++ -- works
14:51:48  -- Detecting CXX compiler ABI info
14:51:48  -- Detecting CXX compiler ABI info - done
14:51:48  -- Detecting CXX compile features
14:51:48  -- Detecting CXX compile features - done
14:51:48  -- Check for working C compiler: /usr/lib/ccache/cc
14:51:48  -- Check for working C compiler: /usr/lib/ccache/cc -- works
14:51:48  -- Detecting C compiler ABI info