- Fix use of deprecated `exec_program`.
- Reformat 'UninstallConky.cmake.in' file.
- Separate BUILD_TESTS (now BUILD_TESTING to shadow CTest variable) from
MAINTAINER_MODE flag.
- Remove BUILD_TESTS flag from CI, default it already true.
- Add support for (s)ccache, enable it by default.
- Add REPRODUCIBLE_BUILD to disable (s)ccache default.
- Set REPRODUCIBLE_BUILD=ON in nix flake.
- Add sccache to CI.
- Add CI detection for possible future use, stored in ENV_IS_CI variable.
- Minor formatting tweaks.
Signed-off-by: Tin Švagelj <tin.svagelj@live.com>